Success exit from settag.php if desired state already exists

If you're adding a tag that already exists, or deleting a tag that
doesn't exist, using settag.php, the exit value is 0 instead of
previous -1. This makes scripting around tags a wee bit easier.

scripts/settag.php

@@ -66,12 +66,12 @@ if ($i !== false) {
         }
     } else {
         print "Already set.\n";
-        exit(-1);
+        exit(0);
     }
 } else {
     if (have_option('d', 'delete')) { // Delete
         print "No such tag.\n";
-        exit(-1);
+        exit(0);
     } else {
         $tags[] = $tag;
         $result = $sn->setTags($tags);