Add rxgk_crypto, wrappers for RFC3961 operations

Currently only linked into rxgkd, but will be needed by both client
and server eventually.

Add LIB_krb5 and -lk5crypto as library dependencies.

Change-Id: I4f107c80c1df2342150f203ff4960b3464d6a6a6
Make an actual token

Encrypt and encode it, too.