netgraph: teach ngctl to attach and run itself in a jail

Add -j <jail> flag to ngctl to allow ngctl to attach and run inside
a jail. This allow parent to manipulate netgraph nodes in the jail
even if ngctl is not available.

Submitted by: David Marker <dave_freedave.net>
Reviewed by: kevans, zlei, jamie
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D50241