server: Use really the negotiated signature type

Generally, when the extension negotiation is enabled and client supports
SHA2 algorithms for RSA, they are supposed to be prioritized against the
old SHA1. If it is not (ssh-rsa is listed in front of rsa-sha2-* hostkey
algorithms during negotiation), the server wrongly tries to provide the
new typo of signature, ignoring the negotiated algirithm

This commit propagates the digest algorithm from negotiation to the actual
signature functions, which were previously responsible for decision
about the hash algorithm based just on the negotiated extensions.

Fixes T191

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 1ebf506913)

src/wrapper.c

@@ -539,6 +539,7 @@ int crypt_set_algorithms_server(ssh_session session){
 
     method = session->next_crypto->kex_methods[SSH_HOSTKEYS];
     session->srv.hostkey = ssh_key_type_from_signature_name(method);
+    session->srv.hostkey_digest = ssh_key_hash_from_name(method);
 
     /* setup DH key exchange type */
     switch (session->next_crypto->kex_type) {