Add src.conf knob to disable the installation of /var/db/services.db
Default to leaving services.db in place, but allow the removal of the
file and its creation with a src.conf knob.
This file ends up being 2MB in size. For small systems this is a waste
of space but its a tradeoff.
Reviewed by: bdrewery
Differential Revision: https://reviews.freebsd.org/D9655
