allow setting alias when creating bugs and modifying with CLI

The module's modification code already handled aliases, but the
CLI didn't. Neither the module nor the CLI handled aliases at
bug creation, but it seems to have been in the API since 3.2
or earlier.

(crobinso: add tests)
tests: Fix tests with latest bugzilla values
Clarify rhbz support for Bug.update alias
Bugzilla.logged_in now returns live state

- since BugZilla XMLRPC does not currentlly expose a way to query login state
  for a user, we workaround this by calling User.get() with ids set
- extend test suite to cover Bugzilla.logged_in
Add comments around using User.valid_login for Bugzilla 5 and above in the future.