Support for detached certificates in userauth.

This is triggered by a new config option, or alternatively a -cert
command-line option. You provide a certificate file (i.e. a public key
containing one of the cert key formats), and then, whenever you
authenticate with a private key that matches the public key inside
that certificate, the certificate will be sent to the server in place
of whatever public key it would have used before.

I expect this to be more convenient for some users than the approach
of baking the certificate into a modified version of the PPK file -
especially users who want to use different certificates on the same
key, either in sequence (if a CA continually reissues certificates
with short lifetimes) or in parallel (if different hosts trust
different CAs).

In particular, this substitution is applied consistently, even when
doing authentication via an agent. So if your bare private key is held
in Pageant, you can _still_ specify a detached certificate, and PuTTY
will spot that the key it's picked from Pageant matches that
certificate, and do the same substitution.

The detached certificate also overrides an existing certificate, if
there was one on the public key already.

putty.h

@@ -1780,6 +1780,7 @@ NORETURN void cleanup_exit(int);
     X(BOOL, NONE, change_username) /* allow username switching in SSH-2 */ \
     X(INT, INT, ssh_cipherlist) \
     X(FILENAME, NONE, keyfile) \
+    X(FILENAME, NONE, detached_cert) \
     /* \
      * Which SSH protocol to use. \
      * For historical reasons, the current legal values for CONF_sshprot \