pthread_atfork: fix return value on malloc failure

POSIX requires pthread_atfork to report errors via its return value,
not via errno. The only specified error is ENOMEM.

src/thread/pthread_atfork.c

@@ -1,4 +1,5 @@
 #include <pthread.h>
+#include <errno.h>
 #include "libc.h"
 #include "lock.h"
 
@@ -34,7 +35,7 @@ void __fork_handler(int who)
 int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void))
 {
 	struct atfork_funcs *new = malloc(sizeof *new);
-	if (!new) return -1;
+	if (!new) return ENOMEM;
 
 	LOCK(lock);
 	new->next = funcs;