Alterations:
- 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; see _sh_fork()
  for a working example that doesn't lock up or produce segfaults.
  In this commit I duplicate _sh_fork()'s behavior in spawnveg
  for the fork fallback and with POSIX_SPAWN_SETSIGDEF for the
  posix_spawn version.
- 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 isn'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.