sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

net: clean up empty lines in .c and .h files

sys: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

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

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

MFC r207680:

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

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

- Import the HEAD csup code which is the basis for the cvsmode work.

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.