Motivation is to simplify testing netgraph within jails.
This also lets you use netgraph with jails that were built with either `WITHOUT_NETGRAPH` or `WITHOUT_NETGRAPH_SUPPORT`.
That is `ngctl -j jid_or_name ...` is able to run even when `jexec jid_or_name ngctl ...` may not be able to (when ngctl(8) is not present in the jail).
ngctl(8) can NOT load modules when used with `-j` option. This is good and no different from ifconfig(8) `-j` option for epair(4).
I don't want to be disingenuous, it does makes testing easier as I can use a "bare jail" and not care if ngctl(8) is available. But the reason I want it for a separate review I will be making available soon.
This is the only patch of the series that changes existing code.
Similar reviews (D40213 and D40377) asked for the jail_attach to be broken out of the getopt handling. I did not do that because the '-f' option already just opens a file in place.