Support SSH signatures

https://www.agwa.name/blog/post/ssh_signatures

See here for more details:
https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.sshsig
https://github.com/openssh/openssh-portable/blob/master/sshsig.c
2a9c9f7272

libagent/ssh/protocol.py

@@ -144,6 +144,10 @@ class Handler:
             signature = self.conn.sign(blob=blob, identity=key['identity'])
         except IOError:
             return failure()
+        except Exception:
+            log.exception('signature with "%s" key failed', label)
+            raise
+
         log.debug('signature: %r', signature)
 
         try: