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

To avoid namespace polution in NetBSD:

``struct descriptor'' -> ``struct fdescriptor''

$Id$ -> $FreeBSD$

When resending chap challenges, resend the same challenge
each time rather than making up a new one.

Increase the authname/authkey max sizes to 100 characters.

Allow ``authkey'' specifications beginning with ``!''.
When a challenge is received, the text following the
``!'' is executed as a program (expanding stuff in the same
way that ``sh'' and ``!bg'' do). The program is passed the
peer name, peer challenge and local ``authname'' on standard
input and is expected to output the name/key combination that
should be used to build the CHAP response.

This provides support for Secure ID cards (guess what I was
given at work recently!) using CHAP.

Examples will follow.

Decouple pap & chap output routines from the corresponding
input routines and take advantage of the new init/continue
interface in libradius. This allows a timely response on
other links in an MP setup while RADIUS requests are in
progress as well as the ability to handle other data from
the peer in parallel. It should also make the future addition
of PAM support trivial.

While I'm in there, validate pap & chap header IDs if
``idcheck'' is enabled (the default) for other FSM packet
types.

NOTE: This involved integrating the generation of chap
challenges and the validation of chap responses
(and commenting what's going on in those routines).
I currently have no way of testing ppps ability
to respond to M$Chap CHALLENGEs correctly, so if
someone could do the honours, it'd be much
appreciated (it *looks* ok!).

Sponsored by: Internet Business Solutions Ltd., Switzerland

o If we come out of select() with only write descriptors that
end up writing zero bytes, sleep for 1/10 of a second so that
we don't end up using up too much cpu.
This should only ever happen on systems that wrongly report a
descriptor as writable despite the tty buffer being full.
Discussed with: Jeff Evarts

o Do an initial run-time check to see if select() alters the passed
timeval. This knowledge isn't yet used, but will be soon.

o Move our prompt descriptor list outside of the bundle.
It's now dealt with by the `server' object. This simplifies
things as we only have one list of prompt descriptors and
the log_ routines check prompt::logactive to determine
whether it should be used for output.
o Include the MP socket UpdateSet() result in bundle::UpdateSet().
o Don't select on the tun device unless we're in NETWORK
phase or AUTO mode.
o Stop the idle timer when we go to DEAD phase. We may
have transferred a link and not had a chance to kill
it.
o Don't fail when trying to unlink our transferred datalink
from our descriptor lists just before the transfer.
o Add our link descriptor to the write set if we got a short
write the last time (physical::out is set).
o Log the connection source address when a connection is closed.
o Remove descriptor::next field. Descriptor lists are not required
any more.

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