MFC r342845,342846: Port NetBSD improvements:

- Add -l support for xz files
- Add lzip support to gzip based on the example lzip decoder.

Obtained from: NetBSD
Relnotes: yes

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.

MFC r326052: Support SIGINFO.

Obtained from: NetBSD

MFC r313327:

Reflect actual NetBSD revision we already have.

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

According to the NetBSD foundation [1]:

Third parties are encouraged to change the license on any files which have
a 4-clause license contributed to the NetBSD Foundation to a 2-clause
license. We would also encourage you to inform us about these files, so
that we can continue to track the many places in which NetBSD is used.

http://www.netbsd.org/about/redistribution.html#why2clause [1]

Requested by: joel@

Incorporate recent changes from NetBSD. Most notable change is the addition
of support of decompressing xz files.

Obtained from: NetBSD