FreeBSD support enhanced

  Summary of changes:

    - properly react to changes done by apxs when it activates apache
      modules, so httpd.conf gets reverted to our desired state after
      a module is installed (FreeBSD),
    - install apache port (FreeBSD) appropriate for the MPM module
      selected by user,
    - more MPM modules under (FreeBSD),
    - new class apache::package to intelligently install appropriate
      apache package (depending on mpm_module chosen),
    - elaborated $apache::params::mod_packages list,
    - other fixes,