Add signal mediation where the signal can be mediated based on the
signal, direction, or the label or the peer/target. The signal perms
are verified on a cross check to ensure policy consistency in the case
of incremental policy load/replacement.
The optimization of skipping the cross check when policy is guaranteed
to be consistent (single compile unit) remains to be done.
policy rules have the form of
SIGNAL_RULE = [ QUALIFIERS ] 'signal' [ SIGNAL ACCESS PERMISSIONS ]
[ SIGNAL SET ] [ SIGNAL PEER ]
SIGNAL ACCESS PERMISSIONS = SIGNAL ACCESS | SIGNAL ACCESS LIST
SIGNAL ACCESS LIST = '(' Comma or space separated list of SIGNAL
ACCESS ')'