With the source port also set, we only need to enable inbound
firewall rules on the 'server' side of the connection, as
the conntrack will allow replies.
Source/Dest ports are required for the nebula firewall on the
receiving side, allow the port to be configured so that it can
be matched to specific rules as required.
This commit augments the Control API by providing new methods to
inject message packets destined peer nodes, and/or to intercept
message packets of a custom message subtype that are received from
peer nodes.
