In addition to the 'M' and 'O' scripts (for when 'Managed' and 'Other'
flags are set) also introduce an 'always' script that is called for any
router advertisement (so even if M and O are not set).
This is primarly useful for systems like pfSense that wish to be
informed of routers for further system configuration.
See also https://redmine.pfsense.org/issues/14072
Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39931
476babaea7