As it stands right now, the default devfs rulesets are only loaded as a

As it stands right now, the default devfs rulesets are only loaded as a
side effect of something else using them. If they haven't been loaded
already but you want to use them, say for configuring a jail, you're out
of luck.

So add a knob to always load the default rulesets. While I'm here document
the other devfs_ knobs in rc.conf.5.