Major restructuring of big privatemsg_page into separate menu callbacks

and form validate/submit functions.  More code cleanup.
Functional changes:
- do OPTIMIZE in cron for mysqli too
- omit locked users in autocomplete
- add watchdog messages on attempts to use unauthorized folders
- after delete of message being viewed, return to folder it was in (was always inbox)
- make List messages in msg view link to containing folder (was always inbox)