New functions using struct nettle_hash.
(make_hmac_algorithm): Reorganized hmac implementation again. Now
uses struct nettle_hash.
Updated all users.

Rev: src/abstract_crypto.c:1.20
Rev: src/dh_exchange.c:1.14
Rev: src/encrypt.c:1.24
Rev: src/keyexchange.c:1.80
Rev: src/keyexchange.h:1.52
Rev: src/lsh.c:1.158
Rev: src/lshd.c:1.134
Rev: src/pkcs5-test.c:1.6
Rev: src/pkcs5.c:1.6
Rev: src/publickey_crypto.h:1.45
Rev: src/read_packet.c:1.51
Rev: src/server_authorization.c:1.18
Rev: src/server_authorization.h:1.5
Rev: src/sexp-conv.c:1.13
Rev: src/spki.c:1.38
Rev: src/spki.h:1.25
Rev: src/srp-gen.c:1.14
Rev: src/srp.h:1.5
Rev: src/srp_exchange.c:1.13