Support glibc 2.35's posix_spawn_file_actions_addtcsetpgrp_np(3)

This commit implements support for the glibc 2.35
posix_spawn_file_actions_addtcsetpgrp_np(3) extension[2][3],
updating spawnveg(3) to use the new function for setting the
terminal group. This was done with the intention of improving
performance in interactive shells without reintroducing previous
race conditions[4][5].

[1]: https://sourceware.org/pipermail/libc-alpha/2022-February/136040.html
[2]: https://sourceware.org/git/?p=glibc.git;a=commit;h=342cc934
[3]: https://sourceware.org/git/?p=glibc.git;a=commit;h=6289d28d
[4]: https://github.com/ksh93/ksh/issues/79
[5]: https://www.mail-archive.com/ast-developers@research.att.com/msg00717.html

src/cmd/ksh93/sh/path.c:
- Tell spawnveg(3) to set the terminal process group when launching
  a child process in an interactive shell.

src/cmd/ksh93/sh/xec.c:
- If posix_spawn_file_actions_addtcsetpgrp_np(3) is available,
  allow use of spawnveg(3) (via sh_ntfork()) even with job control
  active.
- sh_ntfork(): Reimplement most of the SIGTSTP handling code
  removed in commit 66c37202.

src/lib/libast/comp/spawnveg.c,
src/lib/libast/misc/procopen.c,
src/lib/libast/features/sys:
- Add support for posix_spawn_file_actions_addtcsetpgrp_np(3).
- Allow spawnveg to set the terminal process group when pgid == 0.
  This was necessary to avoid race conditions when using the new
  function.

src/lib/libast/features/lib:
- Detect posix_spawn_file_actions_addtcsetpgrp_np(3).
- Do not detect an OS spawnveg(3). With the API changes to spawnveg
  in this pull request ksh probably can't use the OS's spawnveg
  function anymore. (That's assuming anything else even provides a
  spawnveg function to begin with, which is unlikely.)

src/lib/libast/features/api,
src/cmd/ksh93/include/defs.h:
- Bump libast version (20220101 => 20220201) due to the spawnveg(3)
  API change.

src/lib/libast/man/spawnveg.3:
- Document the changes to spawnveg(3) in the corresponding man
  page. Currently, it will only use the new tcfd argument if
  posix_spawn_file_actions_addtcsetpgrp_np(3) is supported. This
  could also be implemented for the fork(2) fallback, but for now
  I've avoided changing that since actually using it in the fork
  code would likely require a lot of hackery to avoid attempting
  tcsetpgrp with vfork (the behavior of tcsetpgrp after vfork is
  not portable) and would only benefit systems that don't have
  posix_spawn and vfork (I can't recall any off the top of my head
  that would fall under that category).
- Updated the man page to account for spawnveg's change in
  behavior.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/lib/libast/features/sys

@@ -195,7 +195,7 @@ extern	setpgid		int		(pid_t, pid_t)
 extern	setsid		pid_t		(void)
 extern	setuid		int		(uid_t)
 extern	sleep		unsigned		(unsigned int)
-extern	spawnveg	pid_t		(const char*, char* const[], char* const[], pid_t)
+extern	spawnveg	pid_t		(const char*, char* const[], char* const[], pid_t, int)
 extern	srand		void		(unsigned int)
 extern	strcasecmp	int		(const char*, const char*)
 extern	strcat		char*		(char*, const char*)