rc.subr: remove the dependency on bsdconfig

rc.subr uses sysrc(8) for the 'enable' and 'disable' commands, which
means the entire rc(8) stack depends on bsdconfig. Instead, provide a
minimal amount of rc.conf-editing functionality in rc.subr and use it to
implement these commands.

Reviewed by: des, bapt
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50325