For some reason execenv::exec() isn't successfully marked noreturn
(rlibby reports that virtual functions can't be noreturn), but calling
methods are so gcc rightly complains.  Work around this by adding
explicit __builtin_unreachable() calls.

Reviewed by:	imp, markj
Fixes:		257e70f1d5ee kyua: Add FreeBSD Jail execution environment support
Differential Revision:	https://reviews.freebsd.org/D46041

A new Kyua concept is added -- "execution environment". A test can be
configured to be run within a specific environment. The test case
lifecycle is extended respectively:
- execenv init (creates a jail or does nothing for default
  execenv="host")
- test exec
- cleanup exec (optional)
- execenv cleanup (removes a jail or does nothing for default
  execenv="host")

The following new functionality is provided, from bottom to top:

1 ATF based tests

- The new "execenv" metadata property can be set to explicitly ask for
  an execution environment: "host" or "jail". If it's not defined, as
  all existing tests do, then it implicitly means "host".

- The new "execenv.jail.params" metadata property can be optionally
  defined to ask Kyua to use specific jail(8) parameters during creation
  of a temporary jail. An example is "vnet allow.raw_sockets".

  Kyua implicitly adds "children.max" to "execenv_jail_params"
  parameters with the maximum possible value. A test case can override
  it.

2 Kyuafile

- The same new metadata properties can be defined on Kyuafile level:
  "execenv" and "execenv_jail_params".

- Note that historically ATF uses dotted style of metadata naming, while
  Kyua uses underscore style. Hence "execenv.jail.params" vs.
  "execenv_jail_params".

3 kyua.conf, kyua CLI

- The new "execenvs" engine configuration variable can be set to a list
  of execution environments to run only tests designed for. Tests of not
  listed environments are skipped.

- By default, this variable lists all execution environments supported
  by a Kyua binary, e.g. execenvs="host jail".

- This variable can be changed via "kyua.conf" or via kyua CLI's "-v"
  parameter. For example, "kyua -v execenvs=host test" will run only
  host-based tests and skip jail-based ones.

- Current value of this variable can be examined with "kyua config".

[markj] This feature has not landed upstream yet.
See the discussion in https://github.com/freebsd/kyua/pull/224 .
Having the ability to automatically jail tests allows many network tests
to run in parallel, giving a drastic speedup.  So, let's import the
feature and start using it in main.

Signed-off-by:  Igor Ostapenko <pm@igoro.pro>
Reviewed by:    markj, kp
Tested by:      markj, kp
MFC after:	3 months
Differential Revision:  https://reviews.freebsd.org/D45865

Having kyua in the base system will simplify automated testing in CI and
eliminates bootstrapping issues on new platforms.

The build of kyua is controlled by WITH(OUT)_TESTS_SUPPORT.

Reviewed by:	emaste
Obtained from:	CheriBSD
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D24103