MFC r345318:

lockf(1): return EX_UNAVAILABLE if -n is used and the lock file does not
exist

Apply EX_UNAVAILABLE patch part from PR 170775 to match the documentation.

Was checked with a command from PR 210770:
lockf -n /tmp/doesnotexist echo; echo $?

PR:		210770

usr.bin/lockf/lockf.c

@@ -174,6 +174,8 @@ acquire_lock(const char *name, int flags)
 	if ((fd = open(name, O_RDONLY|O_EXLOCK|flags, 0666)) == -1) {
 		if (errno == EAGAIN || errno == EINTR)
 			return (-1);
+		else if (errno == ENOENT && (flags & O_CREAT) == 0)
+			err(EX_UNAVAILABLE, "%s", name);
 		err(EX_CANTCREAT, "cannot open %s", name);
 	}
 	return (fd);