Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Updates for netgraph node manual pages.

Discussed with: glebius
Submitted by: Mamontov Roman <mr.xanto@gmail.com>

Add an optional "persistent" flag to ng_hub and ng_bridge, which if set,
disables automatic node shutdown when the last hook gets disconnected.

Reviewed by: julian

A simple packet distribution node type that acts like an Ethernet hub.