global: Use mail_user_deinit() wherever possible

global: Use mail_user_deinit() wherever possible

This makes sure that the user is fully deinitialized in the places where we
expect it to be.

This mainly makes sure that lmtp won't continue running code for the user
after lmtp has already switched to running as root. That could then end up
reading/writing files as root. This can happen only if there are bugs in
the code that leaks user references. Normally user is supposed to be fully
unreferenced before switching to root, so this change just adds an assert
to make sure it is.