Use posix_spawn if possible.

Use posix_spawn if possible.

posix_spawn is less error-prone than vfork + execve, and can make
better use of system-specific enhancements like 'clone' on Linux.  Use
it if we don't need to configure a pseudoterminal.

Backported from commit a60053f8368e058229721f1bf1567c2b1676b239.
Unlike that commit, only define USABLE_POSIX_SPAWN on macOS, because
there posix_spawn is much faster than vfork.

Don't merge to master.

* configure.ac (HAVE_SPAWN_H, HAVE_POSIX_SPAWN)
(HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR)
(HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR_NP)
(HAVE_POSIX_SPAWNATTR_SETFLAGS, HAVE_DECL_POSIX_SPAWN_SETSID): New
configuration variables.
* src/callproc.c (USABLE_POSIX_SPAWN): New configuration macro.
(emacs_posix_spawn_init_actions)
(emacs_posix_spawn_init_attributes, emacs_posix_spawn_init): New
helper functions.
(emacs_spawn): Use posix_spawn if possible.

(cherry picked from commit a60053f8368e058229721f1bf1567c2b1676b239)