Remove support for libmhash/libmcrypt.

libmcrypt seems to dead, maintainer address bounces,
and cast-128 fails on 2 of the 3 test vectors from RFC2144.

So I see no reason to keep around stuff I don't trust
anymore.

Support for several crypto libraries is probably only
confusing to users, although it was good for initial
developing - it helped to find hidden assumptions and
forced me to create regression tests for all functionality.

Marko Kreen