ntb_hw(4): Add sysctls for administrative/test link config, state

dev.ntb_hw.0.admin_up=0/1: Like ifconfig UP/DOWN.
dev.ntb_hw.0.active=0/1: Like ifconfig 'status'

Reviewed by: ngie
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D6429