[gdb] Add c_ctrl/c_unctrl

Readline exports macros CTRL/UNCTRL (compatible with readline macro
CTRL_CHAR), such that:
- CTRL_CHAR ('C') == 0
- CTRL_CHAR (0x03 /* ^C */) == 1
- CTRL ('C') == 0x03 /* ^C */
- CTRL ('c') == 0x03 /* ^C */
- UNCTRL (0x03 /* ^C */) == 'C'

Add c_ctrl/c_unctrl, a variant of CTRL/UNCTRL that's compatible with gnulib's
c_iscntrl.

While c_iscntrl (0x7f /* ^? */) == 1, CTRL_CHAR (0x7f /* ^? */) == 0.

Consequently, the current code using CTRL_CHAR also explicitly handles RUBOUT
(which is readline's way of representing ^?).

Use c_iscntrl/c_ctrl/c_unctrl instead of CTRL_CHAR/CTRL/UNCTRL, removing
redundant RUBOUT handling code.

Tested on x86_64-linux.

A v1 was submitted here [2].

Changes in v2:
- change $subject to use gdb instead of gdbsupport since we're adding the
  new functions in gdb/utils.c
- change parameter and result type of c_unctrl to unsigned char
- avoid using readline macros in c_unctrl implementation
- rewrite c_unctrl to be compatible with c_iscntrl, making sure to handle
  ^?.
- add c_ctrl
- use c_ctrl/c_iscntrl instead of CTRL/CTRL_CHAR.

Suggested-By: Tom Tromey <tom@tromey.com> [1]
Approved-By: Tom Tromey <tom@tromey.com>

[1] https://sourceware.org/pipermail/gdb-patches/2026-March/226136.html
[2] https://sourceware.org/pipermail/gdb-patches/2026-March/226171.html