jail: add the -C flag to clean up after a partially removed jail

Differential Revision: https://reviews.freebsd.org/D42670

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

[jail] removal by jid doesn't trigger pre/post stop scripts

This commit fixes bug: command "jail -r" didn't trigger pre/post stop
commands (and others) defined in config file if jid is specified insted of
name. Also it adds basic tests for usr.sbin/jail to avoid regression.

Reviewed by: jamie, kevans, ray
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D21328

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.

Fix up the order in which jail creation processes are run, to preserve
the config file's order in the non-parallel-start case.

PR: 209112
MFC after: 3 days

In preparation for using clang's -Wcast-qual:

Use __DECONST (instead of my own attempted re-invention) for the iov
parameters to jail_get/set(2). Similarly remove the decost-ish hack
from execvp's argv, except the __DECONST is only added at very end.

While I'm at it, remove an unused variable and fix a comment typo.

Improvements in error messages:

Some errors printed the jail name for unnamed (command line) jails.

Attempting to create an already-existing jail from the command line
returned with no error (even for non-root) due to bad logic in
start_state.

Ignore kvm_proc errors, which are typically caused by permission
problems. Instead, stop ignoring permission errors when removing
a jail (but continue to silently ignore other errors, i.e. the
jail no longer existing). This makes non-root attempts at removing
a jail give a clearer error message.

Update copyright dates and other whitespacey stuff.

Change cfstrings from an STAILQ into a TAILQ to allow commands to be
traversed in reverse order.

run_command (mostly) cleanup:

Make the parallelism limit a global instead of always passing it
to run_command and finish_command.
In the case of an empty command string, try to run any other strings
the command may have.
Replace JF_BACKGROUND with its sort-of opposite JF_SLEEPQ.
Change j->comstring earlier to render JF_RUNQ unncessary.
Change the if-else series to a more readable switch statement.
Treat IP_STOP_TIMEOUT like a command, calling run_command which then
calls term_procs.
When the IP_STOP_TIMEOUT "command" finishes, it shouldn't mess with
the parallelism limit.
Make sufficient checks in finish_command and run_command so that
the nonintuitive j->comstring null check isn't necessary to run them.
Rename the "waiting" queue to "depend", because the "sleeping" and
"runnable" queues are also used to wait for something.

Initial work on the new jail(8). There are more features to add, and some
cleaning up to do on existing features, but this is pretty much what the
final product will look like.