Problem:
Nvim crashes on receive SIGTSTP (Ctrl-Z) since 4dabeff308222307ede8e74a2bd341713a7f7d81.

Solution:
* Don't exit on SIGTSTP (not a deadly signal).
* Avoid SIGTSTP handler in os/signal.c.

Co-authored-by: 27Onion Nebell <zzy20080201@gmail.com>

This reverts commit 466f20dd7039e1602c9382f76ac6471d9feb49b7.

Temporary measure for the stable release. Re-enable for nightly after
0.11 release.

Problem:
's 'guicursor' cursor color not working in suckless terminal (ST).
Nvim's builtin terminfo for ST lacks a "Cs" entry, even though ST does
support the cursor color to be set via termcodes.

Solution:
- In `augment_terminfo()`, assume that `st` always supports color cursor.
- Thomas Dickey will add a "Cs" entry for st to ncurses, from which
  Nvim's builtin terminfos are generated.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Ref:
https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h3-Other-buttons

Also remove British National Replacement Character Set.

We keep the DEC Special Graphics and ASCII despite it not being unicode
as some old software such as calcurse still rely on this functionality.

References:
- https://github.com/neovim/neovim/pull/31934#discussion_r1911046426
- https://en.wikipedia.org/wiki/DEC_Special_Graphics
- https://vt100.net/docs/vt220-rm/chapter2.html#S2.4.3

Arrow and function keys do not use CSI u with the kitty keyboard
protocol. For example, the Up arrow key uses CSI A, and the function
keys use a variety of different CSI sequences.

Until now, termkey only parsed subparams used by key events for CSI u
sequences. The result being that any key which did not use CSI u (e.g.
arrow and function keys) was being emitted twice by termkey since it was
not recognizing the separate press and release events.

This commit makes termkey also parse subparams for other key sequences
so that the release key events do not send duplicate keys.

Enable key event reporting in the kitty keyboard protocol. This causes
supporting terminals to send key events for presses, repeats, and key
releases. For now we ignore release events, but eventually we will
support users mapping those.