posix: Add internal symbols for posix_spawn interface

This patch adds internal hidden definition for mostly of the posix_spawn
function so it can be used internally on both popen and system
implementations.

Checked on x86_64-linux-gnu.

	* include/spawn.h (__posix_spawn, posix_spawn_file_actions_addclose,
	__posix_spawn_file_actions_adddup2, __posix_spawn_file_actions_destroy,
	__posix_spawn_file_actions_init, __posix_spawnattr_init,
	__posix_spawnattr_destroy, __posix_spawnattr_setflags,
	__posix_spawnattr_setsigdefault, __posix_spawnattr_setsigmask): New
	prototype.
	* posix/spawn.c (__posix_spawn): Add libc_hidden_def.
	* posix/spawn_faction_addclose.c
	(__posix_spawn_file_actions_addclose): Add hidden definition.
	* posix/spawn_faction_adddup2.c
	(__posix_spawn_file_actions_adddup2): Likewise.
	* posix/spawn_faction_destroy.c
	(__posix_spawn_file_actions_destroy): Likewise.
	* posix/spawn_faction_init.c (__posix_spawn_file_actions_init):
	Likewise.
	* posix/spawnattr_destroy.c (__posix_spawnattr_destroy): Likewise.
	* posix/spawnattr_init.c (__posix_spawnattr_init): Likewise.
	* posix/spawnattr_setdefault.c (__posix_spawnattr_setsigdefault):
	Likewise.
	* posix/spawnattr_setflags.c (__posix_spawnattr_setflags): Likewise.
	* posix/spawnattr_setsigmask.c (__posix_spawnattr_setsigmask):
	Likewise.