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

Fix error in gmountver(8) man page

Also fixes the usage statement for the binary to use the correct terms (provider and geom name, rather than dev and prov, which is incorrect in the latter case)
Sync the man page summary with the new usage statement

PR: 199540
Differential Revision: https://reviews.freebsd.org/D2329
Submitted by: Fabian Keil
Reviewed by: trasz
Approved by: eadler (mentor)
Obtained from: ElectroBSD (original)
MFC after: 1 week
Sponsored by: ScaleEngine Inc.

- Remove gc_argname field. It was introduced for gpart(8), but if I
understand everything correctly, we don't really need it.
- Provide default numeric value as strings. This allows to simplify
a lot of code.
- Bump version number.

Add gmountver, disk mount verification GEOM class.

Note that due to e.g. write throttling ('wdrain'), it can stall all the disk
I/O instead of just the device it's configured for. Using it for removable
media is therefore not a good idea.

Reviewed by: pjd (earlier version)