Implement <spawn.h>.

As described here:

  http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawn.html

And here:

  http://man7.org/linux/man-pages/man3/posix_spawn.3.html

Bug: N/A (but mentioned in my inbox since 2013)
Test: ran tests
Change-Id: I0b27b2919b660779e3bd8a25fb429527c16dc621
diff --git a/libc/include/bits/posix_limits.h b/libc/include/bits/posix_limits.h
index 4038c3a..e5846d6 100644
--- a/libc/include/bits/posix_limits.h
+++ b/libc/include/bits/posix_limits.h
@@ -68,7 +68,7 @@
 #define _POSIX_SEMAPHORES _POSIX_VERSION /* sem_*. */
 #define _POSIX_SHARED_MEMORY_OBJECTS __BIONIC_POSIX_FEATURE_MISSING /* mmap/munmap are implemented, but shm_open/shm_unlink are not. */
 #define _POSIX_SHELL 1 /* system. */
-#define _POSIX_SPAWN __BIONIC_POSIX_FEATURE_MISSING /* <spawn.h> */
+#define _POSIX_SPAWN __BIONIC_POSIX_FEATURE_SINCE(28) /* <spawn.h> */
 #define _POSIX_SPIN_LOCKS __BIONIC_POSIX_FEATURE_SINCE(24) /* pthread_spin_*. */
 #define _POSIX_SPORADIC_SERVER _POSIX_VERSION /* sched_setparam/sched_setscheduler. */
 #define _POSIX_SYNCHRONIZED_IO _POSIX_VERSION