fix missing synchronization of pthread TSD keys with MT-fork

commit 167390f055 seems to have
overlooked the presence of a lock here, probably because it was one of
the exceptions not using LOCK() but a rwlock.

as such, it can't be added to the generic table of locks to take, so
add an explicit atfork function for the pthread keys table. the order
it is called does not particularly matter since nothing else in libc
but pthread_exit interacts with keys.

src/process/fork.c

@@ -37,6 +37,7 @@ static void dummy(int x) { }
 weak_alias(dummy, __fork_handler);
 weak_alias(dummy, __malloc_atfork);
 weak_alias(dummy, __aio_atfork);
+weak_alias(dummy, __pthread_key_atfork);
 weak_alias(dummy, __ldso_atfork);
 
 static void dummy_0(void) { }
@@ -51,6 +52,7 @@ pid_t fork(void)
 	int need_locks = libc.need_locks > 0;
 	if (need_locks) {
 		__ldso_atfork(-1);
+		__pthread_key_atfork(-1);
 		__aio_atfork(-1);
 		__inhibit_ptc();
 		for (int i=0; i<sizeof atfork_locks/sizeof *atfork_locks; i++)
@@ -78,6 +80,7 @@ pid_t fork(void)
 				else **atfork_locks[i] = 0;
 		__release_ptc();
 		if (ret) __aio_atfork(0);
+		__pthread_key_atfork(!ret);
 		__ldso_atfork(!ret);
 	}
 	__restore_sigs(&set);