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

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

Mechanically substitute flags from historic mbuf allocator with
malloc(9) flags within sys.

Exceptions:

- sys/contrib not touched
- sys/mbuf.h edited manually

Mark MALLOC_DEFINEs static that have no corresponding MALLOC_DECLAREs.

This means that their use is restricted to a single C file.

Node constructor methods are supposed to be called in syscall
context always. Convert nodes to consistently use M_WAITOK flag
for memory allocation.

Reviewed by: julian

Properly unref ng_hub nodes on shutdown, so that we don't leak them.

MFC after: 3 days

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

Having moved metadata usage to mbuf tags, remove code that supports
the old way of doing it.

Submitted by: Gleb Smirnoff <glebius@cell.sick.ru>

Don't give up if sending to one link fails, continue.

Suggested by: jmallett

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