Centralise SSH-2 key fingerprinting into sshpubk.c.

There were ad-hoc functions for fingerprinting a bare key blob in both
cmdgen.c and pageant.c, not quite doing the same thing. Also, every
SSH-2 public key algorithm in the code base included a dedicated
fingerprint() method, which is completely pointless since SSH-2 key
fingerprints are computed in an algorithm-independent way (just hash
the standard-format public key blob), so each of those methods was
just duplicating the work of the public_blob() method with a less
general output mechanism.

Now sshpubk.c centrally provides an ssh2_fingerprint_blob() function
that does all the real work, plus an ssh2_fingerprint() function that
wraps it and deals with calling public_blob() to get something to
fingerprint. And the fingerprint() method has been completely removed
from ssh_signkey and all its implementations, and good riddance.

windows/winpgnt.c

@@ -297,7 +297,7 @@ void keylist_update(void)
 	     * nice alignment in the list box, until we encounter a :
 	     * meaning we're into the fingerprint proper.
 	     */
-	    p = skey->alg->fingerprint(skey->data);
+	    p = ssh2_fingerprint(skey->alg, skey->data);
             listentry = dupprintf("%s\t%s", p, skey->comment);
             fp_len = strlen(listentry);
             sfree(p);