Fix regressions in Special:Listusers introduced in r19951 with conversion

Fix regressions in Special:Listusers introduced in r19951 with conversion
to AlphabeticPager class:
* fix PHP notice errors about renamed variable
* preserve group selection across paging and form resubmissions
* fix PHP notice errors about missing variable
* restore the 'username' form variable, was dropped and half-replaced with
  the internal pager offset which behaves differently (eg, you can again
  see a particular user's entry by typing it in the form, whereas under
  r19951 you only got things from the user *after* the one you asked for)