Fix preserve errno in SIGCHLD signal handler

Signal handler may interrupt code that depends on errno, and waitpid()
may modify errno, therefore, errno must be saved and restored before
returning.

src/Mayaqua/Unix.c

@@ -2140,9 +2140,13 @@ void UnixMemoryFree(void *addr)
 // SIGCHLD handler
 void UnixSigChldHandler(int sig)
 {
+	int old_errno = errno;
+
 	// Recall the zombie processes
 	while (waitpid(-1, NULL, WNOHANG) > 0);
 	signal(SIGCHLD, UnixSigChldHandler);
+
+	errno = old_errno;
 }
 
 // Disable core dump