Fix pkill unit tests.

- use daemon(8) to write out a pid file for processes,
  and check for for the existence of that file after
  killing processes
- use explict named parameters to jail(8)


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272305 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f