Clean up wallet encryption code.

Add a new method `DecryptKey` in `crypter.cpp`, that combines the logic for decrypting, initialising and validating a `CKey` object.  This was previously duplicated.

Note that the verification of the public key was not done for `GetKey` before (only while unlocking).  I don't think that it is a problem to check the validity of the key also in `GetKey`, though.
It emphasises that these functions are "internal" to `crypter.cpp`.