Add support for HMAC-SHA-256 and HMAC-SHA-512.

Implement support for these algorithms and wire them up to the libgcrypt
and OpenSSL backends.  Increase the maximum MAC buffer size to 64 bytes
to prevent buffer overflows.  Prefer HMAC-SHA-256 over HMAC-SHA-512, and
that over HMAC-SHA-1, as OpenSSH does.

Closes #40
5 files changed