Fix unsanitised user input vulnerability in GPG.recv_keys().

* Add wrapping of user inputs `keyids` and `keyserver` in calls to
   _fix_unsafe().
 * Apologies for the hairy ''.join([(lambda: x)() for x in keyids]), sort
   of. Common Lisp was my first language and this is totally readable and
   natural to me, and all the rest of you haters just need to get better at
   functional programming.

gnupg.py

@@ -1241,15 +1241,22 @@ class GPG(object):
         >>> assert result
 
         """
+        safe_keyserver = _fix_unsafe(keyserver)
+
         result = self.result_map['import'](self)
-        logger.debug('recv_keys: %r', keyids)
         data = _make_binary_stream("", self.encoding)
-        #data = ""
         args = ['--keyserver', keyserver, '--recv-keys']
-        args.extend(keyids)
+
+        if keyids:
+            if keyids is not None:
+                safe_keyids = ' '.join(
+                    [(lambda: _fix_unsafe(k))() for k in keyids])
+                logger.debug('recv_keys: %r', safe_keyids)
+                args.extend(safe_keyids)
+
         self._handle_io(args, data, result, binary=True)
-        logger.debug('recv_keys result: %r', result.__dict__)
         data.close()
+        logger.debug('recv_keys result: %r', result.__dict__)
         return result
 
     def delete_keys(self, fingerprints, secret=False):