posix: Improve default posix_spawn implementation

posix: Improve default posix_spawn implementation

This patch improves the default posix implementation of posix_spawn{p}
and align with Linux one.  The main idea is to fix some issues already
fixed in Linux code, and deprecated vfork internal usage (source of
various bug reports).  In a short:

   - It moves POSIX_SPAWN_USEVFORK usage and sets it a no-op.  Since
     the process that actually spawn the new process do not share
     memory with parent (with vfork), it fixes BZ#14750 for this
     implementation.

   - It uses a pipe to correctly obtain the return upon failure
     of execution (BZ#18433).

   - It correctly enable/disable asynchronous cancellation (checked
     on ptl/tst-exec5.c).

   - It correctly disable/enable signal handling.

Using this version instead of Linux shows only one regression,
posix/tst-spawn3, because of pipe2 usage which increase total
number of file descriptor.

	* sysdeps/posix/spawni.c (__spawni_child): New function.
	(__spawni): Rename to __spawnix.