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/libc.mips.map b/libc/libc.mips.map
index 3e41f95..16f1209 100644
--- a/libc/libc.mips.map
+++ b/libc/libc.mips.map
@@ -1320,6 +1320,27 @@
     iconv;
     iconv_close;
     iconv_open;
+    posix_spawn;
+    posix_spawnattr_destroy;
+    posix_spawnattr_getflags;
+    posix_spawnattr_getpgroup;
+    posix_spawnattr_getschedparam;
+    posix_spawnattr_getschedpolicy;
+    posix_spawnattr_getsigdefault;
+    posix_spawnattr_getsigmask;
+    posix_spawnattr_init;
+    posix_spawnattr_setflags;
+    posix_spawnattr_setpgroup;
+    posix_spawnattr_setschedparam;
+    posix_spawnattr_setschedpolicy;
+    posix_spawnattr_setsigdefault;
+    posix_spawnattr_setsigmask;
+    posix_spawn_file_actions_addclose;
+    posix_spawn_file_actions_adddup2;
+    posix_spawn_file_actions_addopen;
+    posix_spawn_file_actions_destroy;
+    posix_spawn_file_actions_init;
+    posix_spawnp;
     syncfs;
 } LIBC_O;