Allow rc_conf_files to be redefined in rc.conf(5)

With this change, it's possible to redefine rc_conf_files (e.g.,
sysrc rc_conf_files+=/etc/rc.conf.other) and have the boot process
pick up settings in extra files. The sysrc(8) tool can be used to
query/enumerate/find/manage extra files configured in this manner.

Relnotes: yes