cmirror: Allow explicit empty host

Currently, a local copy with a destination containing ":" doesn't work,
because `cmirror src /tmp/c:d` would try to use "/tmp/c" as a
hostname and `cmirror src :/tmp/c:d` would use ":/tmp/c:d" as a
(local) destination.

Allow explicit empty hostname, so that the second form can be used to
specify a local destination with ":" in the path.

cmirror.c

@@ -1104,7 +1104,7 @@ static void master(char *master_path, char *target) {
     LOCAL_DEV = g_hash_table_new_full(g_int64_hash, g_int64_equal, g_free, NULL);
 
     {
-        g_auto(GStrv) match1 = MATCH_RE("^([^:]+):(.+)$", target);         // system:/path
+        g_auto(GStrv) match1 = MATCH_RE("^([^:]*):(.+)$", target);         // system:/path
         if (match1) {
             slave_path = STEAL_POINTER(&(match1[2]));
             g_auto(GStrv) match2 = MATCH_RE("^([^@]+)@(.+)$", match1[1]);      // user@system