tools: add -n shortcut for --dry-run

Accept -n as a short option for --dry-run in bootctl,
systemd-oomd, systemd-sysusers, and systemd-tmpfiles.

For systemd-repart, make -n equivalent to --dry-run=yes,
while keeping --dry-run=BOOL available.

Follow-up for: 2479f0bb09

src/sysusers/sysusers.c

@@ -2182,7 +2182,7 @@ static int parse_argv(int argc, char *argv[], char ***ret_args) {
                         arg_replace = opts.arg;
                         break;
 
-                OPTION_LONG("dry-run", NULL, "Just print what would be done"):
+                OPTION('n', "dry-run", NULL, "Just print what would be done"):
                         arg_dry_run = true;
                         break;