Add support for dynamically created and destroyed mutexes. This will

Add support for dynamically created and destroyed mutexes.  This will
be needed in some ENGINE code, and might serve elsewhere as well.
Note that it's implemented in such a way that the locking itself is
done through the same CRYPTO_lock function as the static locks.

WARNING: This is currently experimental and untested code (it will get
tested soon, though :-)).