Add rxgk_crypto_rfc3961.c

rxgk wrappers around an external crypto library, in this case, our
in-tree rfc3961 library.  Primitives for encryption/decryption and
MIC/VerifyMIC, ways to generate and free rxgk_key objects, etc..

Change-Id: I7525086043baf54f5c3019b3f5ab3495760c4236
Add RXGK_NOW()

A quick routine to get the current timestamp.

Change-Id: I0051b5c8e5ad61e35431d97454bf2741daba90cb
Add rxgk_token.c

Routines for constructing tokens (both regular and printed), and
helpers.

Change-Id: I0cde7fd0cdf9a27777523cd502b21bdccef41dcc