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
Attempt to make an actual token

Compiles, though note that the name is bogus.

Change-Id: If135597f5c79652e399e6308624adb06dc6286c9
Make release_key a no-op on NULL keys

Change-Id: I703db098c073418992dfc3c91b18bd8c055a9647
Use correct encrypt_length

The krb5 routine returns an error code, not a length.

Change-Id: I0537baf38d29d9756839dde6a797dcd924044db6
Must initialize the displayname

Otherwise the stack garbage makes the structure fail to encode.
Just put nothing in there, for now.

Change-Id: I9f9a28bce97dd063a673899900de3ba69aac0628
Finish up make_wrap_token

There are two encoding stages we need; do both of them.
Also clean up after ourselves.

Change-Id: I6daca0de158664eff3f488ddff8affd4cc1fc650
Fill token identity properly

Change-Id: I7444710d8f258512d29b525cc98007c33bb86c0a