Hi,

As far as I understood, currently `gnutls-hash-mac' does not support nonce
input, so there is no way to properly hash with some MAC algorithms which
require nonce. (e.g AES-GMAC-128)
So I suggest adding an optional argument NONCE to `gnutls-hash-mac' to
support MAC algorithms with nonce.

What I have tested after applying the attached patch are as below.
1. AES-GMC-128/192/256 works correctly.
2. NONCE does not affect SHA256/SHA512 hash results, even if presented.

Since NONCE is added as an optional argument, I believe it will not even
affect existing code using the 'gnutls-hash-mac' function.

Thanks.

Seungki Kim