Add the new functions current_is_above_screen() (which doesn't account
for softwrapped chunks yet, but will when we can scroll edittop partially
off the screen, which is forthcoming), current_is_below_screen() (which
determines whether current[current_x] is past the softwrapped chunk at
the bottom of the screen), and current_is_offscreen() (the union of the
previous two functions).

edit_redraw() and edit_refresh() now use current_is_offscreen() to check
whether they should adjust the viewport, and adjust_viewport() now uses
current_is_above_screen() to determine whether current is on or below
the screen in FLOWING mode.

Use less_than_a_screenful() again, just as in do_uncut_text().

Add the new function less_than_a_screenful() to accomplish this.
It uses go_back_chunks() to count the number of softwrapped chunks
between the end point and the starting point of the paste.

Now softwrap mode and non-softwrap mode behave the same way when
uncutting fewer than editwinrows rows of text.  Accordingly, remove
the call to ensure_line_is_visible(), as it no longer applies.

Use go_forward_chunks() to count softwrapped chunks between the current
cursor position and the bottom of the file.  Now softwrap mode and
non-softwrap mode behave the same way when moving to a line and column
non-interactively, instead of the former's always centering the screen.

These functions, go_back_chunks() and go_forward_chunks(), take a number
of softwrapped chunks (screen rows) to move, a pointer to a buffer, and
a location (specifically, a starting column of a softwrapped chunk).  If
they move successfully, they will update the buffer pointer and location
to point to the beginning of the softwrapped chunk they moved to.

Since non-softwrap mode is effectively just a subset of softwrap mode
in which every line takes up one chunk, these functions also work in
non-softwrap mode.  In this case, their starting column will always be
zero, as it would be in softwrap mode on a line that takes up one chunk.

Nothing uses these functions yet, but that is forthcoming.

And modify a few wordings too.

Most of these variables are freed moments later -- reallocating
them is thus a waste of time.

This addresses a followup of https://savannah.gnu.org/bugs/?50362

.
Reported-by: Mike Frysinger <vapier@gentoo.org>

Our use of globbing (for syntax files) doesn't care about such symlinks.

This avoids an unnecessary increase in size of eight kilobytes -- see
https://lists.gnu.org/archive/html/nano-devel/2017-03/msg00036.html.

Instead of redetermining the entire span of the converted string,
simply move one character left, and then bite it off to make place
for the trailing $.

These functions aren't available on mingw toolchains, so include the
gnulib modules so we can fall back on them.

If the last two columns of a row would be taken up by a double-width
character (and the line is longer than that), don't print it, because
it wouldn't leave any room for the $ character.

This fixes https://savannah.gnu.org/bugs/?50491

.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

It's already in the external documentation; it should be here as well.

The platform's default char type might be signed, which could cause
problems in 8-bit locales.

This addresses https://savannah.gnu.org/bugs/?50289

.
Reported-by: Hans-Bernhard Broeker <HBBroeker@T-Online.de>

This is used by systems that lack sys/wait.h, like Windows.

Windows doesn't have this, so add some build time checks.

Pull in the sigaction module from gnulib, and add ifdef checks
for a bunch of signals that don't exist on Windows.

This is used by systems that lack lstat(), like Windows.

Not all curses implementations provide this function, so drop it
from depending on slang (and assuming ncurses) to a general build
time test.

Windows doesn't have *nix style account databases.

This is used by systems that lack glob.h, like Windows.

Since gnulib provides these now, we can assume them.

The gnulib imports handle this for us now.

Now that we pull in the gnulib getopt-gnu module, we can assume it exists.

Now that we pull in the gnulib regex module, we can assume it exists.

Switch over to gnulib for these.

Switch over to gnulib for these.

These are provided by gnulib now.

On a system with up-to-date GNU tools, this does not change
the size of nano's binary.

We start off with importing only a few modules, although we
don't yet delete the fallback logic for them.

The gettext-0.18.3 release was made in Jul 2013, while the 0.11.5
release was made in Aug 2002.  Time to update :).

Also drop all the bundled gettext m4 files.  When you run autogen.sh,
these will get copied in automatically for you.

The autoconf-2.69 release was made in Apr 2012.
The automake-1.14 release was made in Jun 2013.

Update the requirements so we know we can rely on macros/features
available in those versions.

The mark only needs to be off when calling replace_marked_buffer(),
because this indirectly calls ingraft_buffer(), which fiddles with
the end points if the mark is on.

This fixes https://savannah.gnu.org/bugs/?50415.

Running strlenpt() on a string that takes up more than 80 columns
(the width of an average terminal) takes /more/ time than simply
converting an extra character (the one that will be overwritten
by the "$" at the edge of the terminal).  So... just convert one
more character than necessary when the line is overlong.  In the
most common case, however, the line will fit fully onscreen, and
we save a whole call of strlenpt().

Since we only need span columns of the string, stop scanning the string
as soon as we have that many columns, instead of scanning the string all
the way to the end.  This speeds up the conversion of very long lines.