dtrace tests: Run ksh with -p

In particular, avoid loading the user's .profile file, since that can
have undesirable side effects. Most tests were already careful to do
this.

MFC after: 1 week

Avoid hard-coding a path to ksh in scripts that are executed directly by
the test harness. This is a problem in many of the *.ksh test scripts as
well, but those scripts are executed using a shell whose path is specified
in dtest.pl, so there's no need to modify them.

MFC after: 1 week

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