Don't return but _exit after failed exec*() and fork()

Quoting POSIX[0]:
"Care should be taken, also, to call _exit() rather than exit() if exec cannot be used, since
exit() flushes and closes standard I/O channels, thereby damaging the parent process' standard
I/O data structures. (Even with fork(), it is wrong to call exit(), since buffered data would
then be flushed twice.)"

[0]: http://pubs.opengroup.org/onlinepubs/009695399/functions/vfork.html

nice.c

@@ -44,5 +44,5 @@ main(int argc, char *argv[])
 	savederrno = errno;
 	weprintf("execvp %s:", argv[0]);
 
-	return 126 + (savederrno == ENOENT);
+	_exit(126 + (savederrno == ENOENT));
 }