Add ability to subscribe to 'both' and avoid a deadlock when switching.

Add ability to subscribe to 'both' and avoid a deadlock when switching.

- 'both' means normal and digest versions simultaneously; information about
  this feature is not included in the default list texts, but a few power users
  find it helpful.
- Only take out locks when actually performing a subscription, not before
  checking whether the address is already subscribed; we only really need it
  when we are actually changing the file, and there is a potential deadlock
  which could occur while waiting for unsubscription to complete as part of a
  switch if the lock is taken earlier.
- Also moved code which sends unsubscription confirmations so that it only runs
  once, after unsubsciption has been completed, not every time the address is
  removed; this is only really important when 'both' is a realistic
  subscription option, but it could avoid other double-sends as well.