prevent fork's errno from being clobbered by atfork handlers

If the syscall fails, errno must be set correctly for the caller.
There's no guarantee that the handlers registered with pthread_atfork
won't clobber errno, so we need to ensure it gets set after they are
called.

src/process/fork.c

@@ -18,9 +18,9 @@ pid_t fork(void)
 	__fork_handler(-1);
 	__block_all_sigs(&set);
 #ifdef SYS_fork
-	ret = syscall(SYS_fork);
+	ret = __syscall(SYS_fork);
 #else
-	ret = syscall(SYS_clone, SIGCHLD, 0);
+	ret = __syscall(SYS_clone, SIGCHLD, 0);
 #endif
 	if (!ret) {
 		pthread_t self = __pthread_self();
@@ -31,5 +31,5 @@ pid_t fork(void)
 	}
 	__restore_sigs(&set);
 	__fork_handler(!ret);
-	return ret;
+	return __syscall_ret(ret);
 }