logind: rework delay inhibition logic

- Don't allow any locks to be taken while we are in the process of
  executing the specific operation, so that apps are not surprised if a
  suspend/shutdown happens while they rely on their inhibitor.

- Get rid of the Resumed signal, it was a bad idea, and redundant due to
  PrepareForSleep(false), see below.

- Always send out PrepareFor{Shutdown,Sleep} signals, instead of only if
  a delay lock is taken.

- Move PrepareForSleep(false) after we come back from the suspend, so
  that apps can use this as "Resumed" notification. This also has the
  benefit that apps know when to take a new lock.