MINOR: actions: Add flags to configure the action behaviour

MINOR: actions: Add flags to configure the action behaviour

Some flags can now be set on an action when it is registered. The flags are
defined in the act_flag enum. For now, only ACT_FLAG_FINAL may be set on an
action to specify if it stops the rules evaluation. It is set on
ACT_ACTION_ALLOW, ACT_ACTION_DENY, ACT_HTTP_REQ_TARPIT, ACT_HTTP_REQ_AUTH,
ACT_HTTP_REDIR and ACT_TCP_CLOSE actions. But, when required, it may also be set
on custom actions.

Consequently, this flag is checked instead of the action type during the
configuration parsing to trigger a warning when a rule inhibits all the
following ones.