ike: Treat action_t as flags so 'start' and 'trap' can be combined

ike: Treat action_t as flags so 'start' and 'trap' can be combined

While combining the actions could cause duplicates (while the SA is
initiated, traffic might trigger the trap and the initiation of another
CHILD_SA), the previous commit should avoid most duplicates.  If reuse_ikesa
is disabled, duplicates can't be prevented, though.