[Bug tui/30337] [gdb/tui] TUI in ansi terminal has off-by-one width problem

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30337

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
The problem manifest outside of TUI as well.

Say we have a terminal with:
...
$ echo $COLUMNS
40
...

For contrast, lets start with the terminal in xterm mode, and type 'a' for a
while:
...
$ TERM=xterm gdb -q
         1         2         3         4
1234567890123456789012345678901234567890
(gdb) aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aa
...
All 40 positions in the line are used, before we wrap to the next line.

Now we start gdb with the terminal in ansi mode:
...
$ TERM=ansi gdb -q
         1         2         3         4
1234567890123456789012345678901234567890
(gdb) aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aa
...

The line wraps after 38 chars.

Based on the readline behaviour of hiding one char to deal with the lack of
autowrap, we'd expect wrapping after 39 chars.

The problem is that we start out with COLUMNS=40, then ask readline how wide
the screen is, which replies 39.  Then we use the 39 to set the screensize
using readline, which results in an effective width of 38.

With the patch, we have instead:
...
$ TERM=ansi gdb -q
         1         2         3         4
1234567890123456789012345678901234567890
(gdb) aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aa
...
wrapping after 39 chars, as expected.

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