ifconfig(8): Teach ifconfig to attach and run itself in a jail

Add -j <jail> flag to ifconfig to allow ifconfig to attach and run inside a
jail. This allow parent to configure network interfaces of its children
even if ifconfig is not available in child's tree (e.g. Linux Jails)

Reviewed by: emaste, khng, melifaro
Event: Kitchener-Waterloo Hackathon 202305
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40213