Switch to deterministic DSA.

* agent/pksign.c (rfc6979_hash_algo_string): New.
(do_encode_dsa) [Libgcrypt >= 1.6]: Make use of RFC-6979.

Now that we have a good (and not NSA/NIST demanded ;-) specification
on how to use DSA without a random nonce, we take advantage of it and
thus avoid pitfalls related to a misbehaving RNG during signature
creation.

Note that OpenPGP has the option of using a longer hash algorithm but
truncated to what is suitable for the used DSA key size. The hash
used as input to RFC-6979 will also be one with an appropriate digest
length but not a truncated one. This is allowed by RFC-6979.

• Signed-off-by: Werner Koch <wk@gnupg.org>