posix_spawn_file_actions_addclose: Relax configure test. * m4/posix_spawn.m4 (gl_FUNC_POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE): Test a negative file descriptor, not an out-of-range file descriptor. * tests/test-posix_spawn_file_actions_addclose.c (main): Add comment. * doc/posix-functions/posix_spawn_file_actions_addclose.texi: Update.