<spawn.h>: add posix_spawn_file_actions_addchdir_np()/posix_spawn_file_actions_addfchdir_np().

The recent header nullability additions and the corresponding source
cleanup made me notice that we're missing a couple of actions that most
of the other implementations have. They've also been added to the _next_
revision of POSIX, unchanged except for the removal of the `_np` suffix.

They're trivial to implement, the testing is quite simple too, and
if they're going to be in POSIX soon, having them accessible in older
versions of Android via __RENAME() seems useful. (No-one else has shipped
the POSIX names yet.)

Bug: http://b/152414297
Test: treehugger
Change-Id: I0d2a1e47fbd2e826cff9c45038928aa1b6fcce59
diff --git a/libc/include/spawn.h b/libc/include/spawn.h
index 8bb34d0..6c34b98 100644
--- a/libc/include/spawn.h
+++ b/libc/include/spawn.h
@@ -87,6 +87,9 @@
 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd) __INTRODUCED_IN(28);
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd, int __new_fd) __INTRODUCED_IN(28);
 
+int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, const char* _Nonnull __path) __INTRODUCED_IN(34);
+int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd) __INTRODUCED_IN(34);
+
 __END_DECLS
 
 #endif