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

Add ``enable/disable tcpmssfixup'', defaulting to enabled.

Suggested by: julian
Hijacked from: ru (ports/net/tcpmssd)

Allow a ``timeout secs'' filter option to let specific packet types
effect the idle timer in different ways.

Submitted by: Stefan Esser <se@freebsd.org>

With adjustments by me to document the option in the man page and to
give the same semantics for outgoing traffic as incoming.

I made the style more consistent in ip.c - this should really have
been done as a separate commit.

Handle the availability of TUNSIFHEAD. If it's there, use it.

For the moment this is just overhead, but it'll be used for INET6
support later.

$Id$ -> $FreeBSD$

The protocol must be in network byte order (OpenBSD only).

``struct tunnel_header'' -> u_int32_t in OpenBSD.
struct tunnel_header is going soon.

Suggested by: Theo

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

Add (BSD) copyright headers.

Add id strings to tun.[ch].
Don't try to open ppp.secret if we're never going to use it.

Abstract data read from and written to the tun device,
allowing for a possible header on the front of all packets.
In OpenBSD, there's a structure containing the address
family here.
If we're building under OpenBSD, set up the ``flags'' part
of struct tuninfo (not there under FreeBSD) so that we config
the interface as POINTOPOINT.
Prefix prototypes with ``extern'' in os.c for consistency.

These changes are cosmetic under FreeBSD, but allow ppp to
build & work under OpenBSD (bar the srandomdev() stuff,
the inclusing of <net/if_var.h> and some Makefile symantecs).