Add a function which will perform an HMAC over a set of iovecs,
using the hcrypto provided HMAC functions.
Join contiguous iovecs together before passing them to the hash
function so we make as few calls into the hash as possible.
lib/krb5/crypto.c was a large, monolithic block of code which made
it very difficult to selectively enable and disable particular
alogrithms.
Reorganise crypto.c into individual files for each encryption and
salt time, and place the structures which tie everything together
into their own file (crypto-algs.c)
Add a non-installed library (librfc3961) and test program
(test_rfc3961) which builds a minimal rfc3961 crypto library, and
checks that it is usable.
