[gdb/cli] Add maint info screen

[gdb/cli] Add maint info screen

While working on PRs tui/30337 and cli/30346 I came across various notions of
width in gdb, as reported by gdb, readline, curses and the environment
variables.

As for gdb, readline and the environment variables, the way things work
is:
- Gdb asks readline to detect screen size,
- readline sets the actual screen size in the environment variables
  COLUMNS and LINES,
- readline reports back a screen size to gdb, which may have one column
  less than the actual screen size, to deal with lack of auto-wrap.
  This becomes gdb's notion of screen size (in other words the point where
  we can expect the gdb command line to wrap),
- Gdb then explicitly sets readline's screen size, which readline itself may
  adjust to deal with lack of auto-wrap.  This becomes readlines notion
  of screen size (well, internally the unadjusted one, but it'll report back
  the adjusted one).

Add a command "maint info screen" that prints these notions, both for width
and height.

For TERM=xterm we have:
...
$ TERM=xterm gdb -ex "maint info screen"
Number of characters gdb thinks are in a line is 118.
Number of characters readline reports are in a line is 118.
Number of characters curses thinks are in a line is 118.
Number of characters environment thinks are in a line is 118 (COLUMNS).
Number of lines gdb thinks are in a page is 27.
Number of lines readline reports are in a page is 27.
Number of lines curses thinks are in a page is 27.
Number of lines environment thinks are in a page is 27 (LINES).
...

And for TERM=ansi:
...
$ TERM=ansi gdb -ex "maint info screen"
Number of characters gdb thinks are in a line is 117.
Number of characters readline reports are in a line is 116.
Number of characters curses thinks are in a line is 118.
Number of characters environment thinks are in a line is 118 (COLUMNS).
Number of lines gdb thinks are in a page is 27.
Number of lines readline reports are in a page is 27.
Number of lines curses thinks are in a page is 27.
Number of lines environment thinks are in a page is 27 (LINES).
...

[ The fact that we have "characters readline reports are in a line is 116" is
is due to gdb making readline adjust twice for the lack of auto-wrap, this is
PR cli/30346.

Likewise we can detect tui/30337 by doing a resize in TUI mode and doing
"maint info screen":
...
Number of characters characters curses thinks are in a line is 110.
Number of characters environment thinks are in a line is 111 (COLUMNS). ]

And for TERM=ansi, with width and heigth set to 0:
...
Number of characters gdb thinks are in a line is 4294967295 (unlimited).
Number of characters readline reports are in a line is 32766 (unlimited - 1).
Number of characters curses thinks are in a line is 118.
Number of characters environment thinks are in a line is 118 (COLUMNS).
Number of lines gdb thinks are in a page is 4294967295 (unlimited).
Number of lines readline reports are in a page is 32767 (unlimited).
Number of lines curses thinks are in a page is 27.
Number of lines environment thinks are in a page is 27 (LINES).
...

[ Note that when doing a resize by say maximizing or de-maximizing a terminal,
all reported values are updated, except for curses when not in TUI mode.

Maybe that means there's a bug.  If not, then maybe we should not print
the curses lines unless in TUI mode, or annotate those lines such that it's
clear that the values may be not up-to-date. ]

I'd like to use this command in the regression test for PR cli/30346.

Tested on x86_64-linux.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Tom Tromey <tom@tromey.com>