Girocco/HashUtil.pm: take advantage of Digest::SHA::hmac_sha1
If Digest::SHA::hmac_sha1 can be imported, then use that as the
hmac_sha1 implementation.
It's approximately twice as fast (almost) as the included
hmac_sha1 implementation.
When using the Digest::SHA::hmac_sha1 implementation, double the
maximum key rounds since that's approximately the same CPU cost.
Clients of the HashUtil module automatically get whatever is
implementing hmac_sha1 imported as hmac_sha1 and do not need to
make any changes whatsoever.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>