Treat netname="." in a special way.

Treat netname "." in a special way as if there was no netname
specified.  Before, f.e. tincd -n. -k didn't work as it tried
to open /var/run/tinc-.pid.  Now -n. works as if there was no
-n option is specified.

Signed-Off-By: Michael Tokarev <mjt@tls.msk.ru>

src/tincd.c

@@ -221,7 +221,9 @@ static bool parse_options(int argc, char **argv) {
 				break;
 
 			case 'n':				/* net name given */
-				netname = xstrdup(optarg);
+				/* netname "." is special: a "top-level name" */
+				netname = strcmp(optarg, ".") != 0 ?
+						xstrdup(optarg) : NULL;
 				break;
 
 			case 'o':				/* option */