Signed-off-by: Brand Huntsman <alpha@qzx.com>

This fixes https://savannah.gnu.org/bugs/?52114.

The succeeding character was needed to avoid miscolorings due to nano
getting confused about starts and ends.  But since commit 7ef5c532,
nano should be getting the starts and ends always right, so... undo
"temporary" commit 7b2ea405 from two years ago.

This addresses https://savannah.gnu.org/bugs/?51526.
Reported-by: <exodus6395@googlemail.com>

The new option 'set selectedcolor' applies to marked text, to the
currently selected file in the file browser, and to the highlighted
match during interactive search-and-replace.

Prefer speed above being fully correct.

This addresses https://savannah.gnu.org/bugs/?51644.

Rename the corresponding rc-file option too, of course.

Color it bright green instead, like valid numeric arguments of
'fill' and 'tabsize'.  Also color the argument of 'formatter'.

Extend get_softwrap_breakpoint() to break softwrapped lines on
whitespace when a flag is set.  This flag is controlled by the new
rcfile option "atblanks".  The '>' characters marking two-column
characters at the edge of the screen are disabled when it's on.

If get_softwrap_breakpoint() can't find whitespace in screen range, it
will break the line on the screen edge.  (In this case, a blank can be
on the last column of the screen, but text can't, so that a blank on the
last column doesn't become invisible and possibly break the display.)

This fulfills https://savannah.gnu.org/bugs/index.php?49959

.
Requested-by: Nicholas Boel <axxisd@gmail.com>

Also remove the backslashes from the two syntaxes that contained them.

This completes the fix for https://savannah.gnu.org/bugs/?51370.

The comment marker is not «."» but «.\"», which requires three
backslashes to specify.  Also adjust the documentation.

Negative arguments of 'fill' are bright green since a few commits.
So now color all valid numeric arguments in that same style -- the
yellow was hard to see on a dark background anyway.

Also, color a zero tabsize as invalid.

Most of the keywords listed on the following URL are now highlighted:
  http://php.net/manual/en/reserved.keywords.php

Also color single-quoted strings, and require that //-type comments
are preceded by whitespace or are alone on a line.

With-feedback-from: M <taur@mail.com>

(And in the bargain sort or group some of the keywords.)

This addresses https://savannah.gnu.org/patch/?9342

.
Suggested-by: M <taur@mail.com>

Some .rdf files are XML, but the .rdf extension seems to be used
also for other things.  So... use the 'header' mechanism.

(Hadn't seen that one because of my additional bindings.)

And because I think it looks slightly better.

This avoids applying the default syntax, or the syntax specified
with --syntax, to a ^G help text.

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

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

A tag begins not merely with a "<" but it must be followed by an
ASCII alphabetic character or one of "/", "!" and "?".

Further, color all the valid attribute names in red.

They are not documentation, they are functional elements of nano.