pf: Allow multiple labels to be set on a rule

Allow up to 5 labels to be set on each rule.
This offers more flexibility in using labels. For example, it replaces
the customer 'schedule' keyword used by pfSense to terminate states
according to a schedule.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29936