kyua: Add FreeBSD Jail execution environment support

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