mq: allow qpop if popped revisions are not working dir parents

mq: refactor the pop code to be more readable and allow more changes

merge with crew-stable

Bugzilla 2.18 and on use contrib/sendbugmail.pl, not processmail. During 2.17, Bugzilla ditched the old 'processmail' script. With 2.18 contrib/sendbugmail.pl arrived in its place. For notification emails to work properly, sendbugmail.pl requires as its second parameter the Bugzilla user who made the commit. Otherwise the user will not be recognised as the committer, and will receive notification emails about the commit regardless of their preference about being notified on their own commits. This parameter should be given to processmail also, but wasn't for historical reasons. Add new config with the local Bugzilla install directory, and provide defaults for the notify string which should work for most setups. Still permit notify string to be specified, and for backwards compatibility with any extant notify strings try first interpolating notify string with old-style single bug ID argument. Add new 2.18 support version to introduce sendbugmail.pl. In other words, this update should be backwards-compatible with existing installations, but offers simplified setup in most cases. And as a bonus Bugzilla notification emails will be dispatched correctly; notifiers will not receive an email unless configured to do so.