netgraph/ng_bridge: Introduce "uplink" ports without MAC learning

The ng_bridge(4) node is designed to work in moderately small
environments. Connecting such a node to a larger network rapidly fills
the MAC table for no reason. It even become complicated to obtain data
from the gettable message, because the result is too large to
transmit.

This patch introduces, two new functionality bits on the hooks:

• Allow or disallow MAC address learning for incoming patckets.
• Allow or disallow sending unknown MACs through this hook.

Uplinks are characterized by denied learing while sending out
unknowns. Normal links are charaterized by allowed learning and
sending out unknowns.

Reviewed by: kp
Approved by: kp (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D23963