mbox locking: combine locking functions into one and swap lock order

We used to create a dotlock file first and then lock with fcntl; swap that
order, since locking first with fcntl seems to be more common.

This patch also adds general mbox lock/unlock methods, which call the
dotlock and fcntl-lock methods, and moves the retry logic there.

When the dotlock and fcntl methods fail to acquire a lock, they now raise
a custom exception "LockUnavailable", which gets caught in the general
lock() method.  That way, if we succeed to acquire one lock but fail to
acquire the other, we can release our locks at the upper level and retry.

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