Fix sequence issue with /signup URL form

/signup subpage affects form display and title (just like ?type=signup),
so check for it earlier in execute().
Also rename execute()'s $par parameter to $subPage to match SubPage.php.

Change-Id: Ida615a1039bfa1d8328f5fda24c4e3da66b9d162
Merge "Fix sequence issue with /signup URL form"