… bugs

Alterations:
- Add a version of spawnveg that clones processes with clone(2)
  outright. This method is generally faster than using posix_spawn,
  and is also more portable (since Linux with musl libc *and* NetBSD
  both provide this function). The code was initially based on the
  _real_vfork spawnveg code previously removed in ef9a5c7 (unlike
  vfork(2), there is no problem with running code prior to execve
  for clone with CLONE_VM|CLONE_VFORK).
  - Remove the now dead code implementing tcsetpgrp via
    posix_spawn_file_actions_addtcsetpgrp_np(). This code was only
    ever used with glibc 2.35+. (Not only that, but the only other
    platform that ever considered implementing such a call was
    NetBSD[^1], where it appears to have been abandoned. This is
    ironic considering ksh no longer uses posix_spawn on NetBSD
    as of this commit.)
    This code may be worth visiting if io_uring_spawn()[^2] is ever
    actualized (that project appears to be quite inactive though).
- Bugfix: Don't set the terminal signals to their defaults
  in the parent prior to calling spawnveg. This is the primary
  cause of a lockup in the pty tests which can be reproduced
  as follows:
    $ exec ksh
    $ bin/shtests pty 2>/dev/null
    ^C ^C ^C (SIGINT doesn't work anymore and may segfault)
  The correct way to go about dealing with SIGT* is to set
  those to SIG_DFL in the child process (cf. _sh_fork()).
- Added support for tcsetpgrp to the fork fallback in spawnveg.
  Some form of this appears to have already been attempted in
  AT&T olden times, but that old version was broken and needed
  bugfixes desperately.
  - If the child needs tcsetpgrp, block the terminal signals
    in the parent process via sigcritical(). The posix_spawn
    version doesn't need this because posix_spawn will block
    signals automatically and therefore doesn't need sigcritical.
- Now that the fork fallback for spawnveg works correctly in
  interactive terminals, prefer that to the sh_fork() codepath
  on operating systems without posix_spawn tcsetpgrp support.
  Even though the underlying system call is still ultimately fork,
  the sh_ntfork() codepath is faster than the traditional sh_fork
  codepath. Benchmark:
     $ time /tmp/ksh-devbranch -ic "for i in {1..10000}; do $(whence -p true); done"
     real       0m03.302s
     user       0m00.988s
     sys        0m02.320s
     $ time /tmp/ksh-newspawnveg -ic "for i in {1..10000}; do $(whence -p true); done"
     real       0m03.160s
     user       0m01.187s
     sys        0m01.977s
- To that end, split up the spawnveg versions into spawnveg_fast
  and spawnveg_slow. Choose the appropriate one when spawnveg is
  called; this removes the need for the xec.c ifdef hackery.
  - Removed the ntfork_tcpgrp ifdefs from xec.c; spawnveg can
    handle it by itself now.
- Bugfix: With the spawnveg_fast and spawnveg_slow innovation,
  spawnveg now always has support for setsid. It'll fallback to
  fork if POSIX_SPAWN_SETSID and clone(2) aren't available.
- Bugfix: For the posix_spawn version of spawnveg, the flags should
  be of the short type pursuant to the POSIX specification.
- Optimization: Use pipe2 in the fork fallback for spawnveg when
  it's available to avoid two fcntl syscalls.
- Updated the spawnveg documentation to reflect the new changes.
- _spawnveg(): Restore the terminal process group immediately
  after any failed spawn attempt, rather than only in sh_ntfork().
  - Added a regression test for a crash that occurred because of
    the erroneous tcsetpgrp placement in sh_ntfork().
- path_spawn(): Do not print an error message and longjmp away
  upon encountering E2BIG or some other error; let the calling functions
  take care of that.
- path_exec(): Added handling for E2BIG.
- Fixed a bug that caused sh_ntfork to leak file descriptors
  upon encountering an error (re: 8f848bc). Reproducer:
     #!/bin/ksh
     ls /proc/$$/fd
     redirect 2>/dev/null
     touch /tmp/file
     for i in {1..20}; do
         command /tmp/file <(echo)
     done
     ls /proc/$$/fd
  - Added regression tests for the leak that use 'command' and
    'command -x'.
  - Fixed an old regression test that was giving false negatives.
- Clarified the intent of the sigcrit nesting matches. The ksh2020
  devs appeared to have been confused by this line, so some
  additional clarification explaining the intent should be helpful
  for posterity.
- path.sh: Added tests for the exit status of commands run when forked
  with the & operator.

[^1]: https://blog.netbsd.org/tnf/entry/gsoc_reports_make_system_31
[^2]: https://lwn.net/Articles/908268/