save whole form state, not just QUERY_STRING, for postsignin

save whole form state, not just QUERY_STRING, for postsignin

Normally, needsignin is called when there is a QUERY_STRING, not when a
form is posted. However, it's certianly possible, and should be supported,
to make a form that invokes an ikiwiki action that checks needsignin.

I encountered this when posting ?do=rename&page=foo. The form is displayed
without checking needsignin, for complicated reasons. Posting the form
is when the true authentication happens.