posix_spawn_file_actions_adddup2: Work around Solaris 11 2011-11 bug.

posix_spawn_file_actions_adddup2: Work around Solaris 11 2011-11 bug.

* m4/spawn_h.m4 (gl_SPAWN_H_DEFAULTS): Initialize
REPLACE_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2.
* m4/posix_spawn.m4 (gl_FUNC_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2): New
macro.
* lib/spawn.in.h (posix_spawn_file_actions_adddup2): Test
REPLACE_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2, not REPLACE_POSIX_SPAWN.
* lib/spawn_faction_adddup2.c: Add workaround implementation if
HAVE_WORKING_POSIX_SPAWN.
* modules/spawn (Makefile): Substitute
REPLACE_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2.
* modules/posix_spawn_file_actions_adddup2 (configure.ac): Invoke
gl_FUNC_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2. Test
REPLACE_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2, not REPLACE_POSIX_SPAWN.
(Depends-on): Update conditions.
* doc/posix-functions/posix_spawn_file_actions_adddup2.texi: Mention
the Solaris 11 bug.