srp_format_m2().
(srp_make_reply_msg): Convert secret to string.
(srp_make_client_proof): Compute m2, and return it through an
extra argument..
(srp_process_client_proof): Use srp_format_proofs().
(srp_process_server_proof): Added argument m2. Deleted argument dh.

Rev: src/srp.h:1.4
Rev: src/srp_exchange.c:1.7