sftpd: add support for SSH user certificate authentication

This add support for PROTOCOL.certkeys vendor extension:

https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL.certkeys?rev=1.8

Fixes #117

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

templates/user.html

@@ -65,7 +65,7 @@
             <textarea class="form-control" id="idPublicKeys" name="public_keys" rows="3"
                 aria-describedby="pkHelpBlock">{{range .User.PublicKeys}}{{.}}&#10;{{end}}</textarea>
             <small id="pkHelpBlock" class="form-text text-muted">
-                One public key per line
+                One public key or SSH user certificate per line
             </small>
         </div>
     </div>