policychecker: receiving signals without sender

policychecker: receiving signals without sender

This allows specifying of checking rules for signals without
specifying receive_sender. That is, it allows telling who can
receive some kind of signals without telling who can send them.

Without this change every receive checking rule needed receive_sender field.
This change is analogous to sending signals without send_destination.

Change-Id: Ifb0a596980631315d175b33460ad2579265c1e5e