This fixes https://savannah.gnu.org/bugs/?48988.

These keycodes have non-rebindable entries in the list of shortcuts, so
they will always be there -- no need for a function call to find them.

In the tiny version it would beep and report an unbound key for every
window resize.

Mentioning "GNU nano" instead of "This program" and referring to the
website instead of to a postal address.

Resetting the cutbuffer is irrelevant when 'allow_funcs' is false or
when in view mode, so do_input() can just return in those cases.

And remove a superfluous call.

The prompt has already been trimmed in do_prompt() to fit into the
available space.

Leave the optimization of actual screen writes to ncurses.

Add the keycodes and routines to allow the user to forego setting the
mark explicitly (with M-A / ^6) and instead quickly select a few words
or lines by holding down Shift together with the movement keys.

(Some combinations with Shift are swallowed by some terminal emulators.
To work around some of those, the combinations Shift+Alt+Left/Right work
as Shift+Home/End and Shift+Alt+Up/Down work as Shift+PageUp/PageDown.)

This avoids showing a ghost item in, for example, the WriteOut menu
when not using --tempfile and the user does not bind 'discardbuffer'.

This fixes https://savannah.gnu.org/bugs/?48917.

This fixes https://savannah.gnu.org/bugs/?48916.

The $ sign looks too much like an S, and it "obscures" the answer by
being so "massive".  The < and > signs are lighter and work better.

This fixes https://savannah.gnu.org/bugs/?48894.

This fixes https://savannah.gnu.org/bugs/?48912.

This fixes https://savannah.gnu.org/bugs/?48796.

This fixes https://savannah.gnu.org/bugs/?48792.

And move a statement to a better place.

This also prevents the message from spilling into the help lines
when the terminal is less than four columns wide.  (Not that one
can read anything then, but it looks better.)

This fixes https://savannah.gnu.org/bugs/?48852.

Even when the edit window consists of just one or two lines, the PageUp
and PageDown functions (^Y and ^V) should continue to move the window.

This fixes https://savannah.gnu.org/bugs/?48805.

This fixes https://savannah.gnu.org/bugs/?48787.