posix: Fix posix_spawnp to not execute invalid binaries in non compat mode (BZ#23264)

posix: Fix posix_spawnp to not execute invalid binaries in non compat mode (BZ#23264)

Current posix_spawnp implementation wrongly tries to execute invalid
binaries (for instance script without shebang) as a shell script in
non compat mode.  It was a regression introduced by
9ff72da471a509a8c19791efe469f47fa6977410 when __spawni started to use
__execvpe instead of __execve (glibc __execvpe try to execute ENOEXEC
as shell script regardless).

This patch fixes it by using an internal symbol (__execvpex) with the
faulty semantic (since compat mode is handled by spawni.c itself).

It was reported by Daniel Drake on libc-help [1].

Checked on x86_64-linux-gnu and i686-linux-gnu.

	[BZ #23264]
	* include/unistd.h (__execvpex): New prototype.
	* posix/Makefile (tests): Add tst-spawn4.
	(tests-internal): Add tst-spawn4-compat.
	* posix/execvpe.c (__execvpe_common, __execvpex): New functions.
	* posix/tst-spawn4-compat.c: New file.
	* posix/tst-spawn4.c: Likewise.
	* sysdeps/unix/sysv/linux/spawni.c (__spawni): Do not interpret invalid
	binaries as shell scripts.
	* sysdeps/posix/spawni.c (__spawni): Likewise.

[1] https://sourceware.org/ml/libc-help/2018-06/msg00012.html