rtsol: introduce an 'always' script

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