MFC r326276:

various: 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.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Re-implement LQM, this time according to the rfc.

PR: 11293
MFC after: 4 weeks

Calculate the average link throughput using a counter based on the
cumulative total of all active links rather than basing it on the
total of PROTO_MP traffic.

This fixes a problem whereby Cisco routers send PROTO_IP packets only
when there's only one link (hmm, what a good idea!).

Cosmetic: Make struct mbuf more like kernel mbufs.

o Split the two IPCP queues into three - one for FSM data
(LCP/CCP/IPCP), one for urgent IP traffic and one for
everything else.
o Add the ``set urgent'' command for adjusting the list of
urgent port numbers. The default urgent ports are 21, 22,
23, 513, 514, 543 and 544 (Ports 80 and 81 have been
removed from the default priority list).
o Increase the buffered packet threshold from 20 to 30.
o Report the number of packets in the IP output queue and the
list of urgent ports under ``show ipcp''.

$Id$ -> $FreeBSD$

Add ``show layers'' to see the protocol layering for a link.

o Redesign the layering mechanism and make the aliasing code part of
the layering.

We now ``stack'' layers as soon as we open the device (when we figure
out what we're dealing with). A static set of `dispatch' routines are
also declared for dealing with incoming packets after they've been
`pulled' up through the stacked layers.

Physical devices are now assigned handlers based on the device type
when they're opened. For the moment there are three device types;
ttys, execs and tcps.

o Increment version number to 2.2
o Make an entry in [uw]tmp for non-tty -direct invocations (after
pap/chap authentication).
o Make throughput counters quad_t's
o Account for the absolute number of mbuf malloc()s and free()s in
``show mem''.
o ``show modem'' becomes ``show physical''.

If we've got a full output buffer queue and cannot send
anything for two mintues (see ``set choked'' and ``show
bundle''), nuke the ip, mp and link level buffer queues.

This should fix problems where ``ppp -auto'' seems to stop
responding after failing to connect to the peer a few times.

o Make sure we adjust our min seq and process any outstanding queued
incoming fragments when a link goes down.
o Don't use the minimum sequence numbers of links that aren't open.
o Understand sequence number wrapping when determining the minimum
sequence number.
o Add & adjust a few comments.

MFMP: Make ppp multilink capable.
See the file README.changes, and re-read the man page.