mqueue: Introduce kern_kmq_timedreceive & kern_kmq_timedsend

Reviewed by: imp, kib
Pull Request: https://github.com/freebsd/freebsd-src/pull/1248

mqueue: Export some functions to be used by Linuxulator

Reviewed by: imp, kib
Pull Request: https://github.com/freebsd/freebsd-src/pull/1248

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

sys/sys: further 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.

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

- Import the HEAD csup code which is the basis for the cvsmode work.

Last step to make mq_notify conform to POSIX standard, If the process
has successfully attached a notification request to the message queue
via a queue descriptor, file closing should remove the attachment.

Bring in mqueue.h to define struct mq_attr.