MFC r339971: Add new rc keywords: enable, disable, delete

This adds new keywords to rc/service to enable/disable a service's
rc.conf(5) variable and "delete" to remove the variable.

When the "service_delete_empty" variable in rc.conf(5) is set to "YES"
(default is "NO") an rc.conf.d file (in /etc/ or /usr/local/etc) is
deleted if empty after modification using "service $foo delete".

Submitted by: lme (modified)
Reviewed by: 0mp (previous version), lme, bcr
Approved by: re (rgrimes)
Relnotes: yes
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D17113