Add pkg_script support

This commit allows you to manage the service installed via pkg_add. e.g.:

    service 'bird' do
      action [:enable, :start]
      parameters(:pkg_script => true)
    end

pkg_script variables are stored at '/etc/pkg_scripts.conf' and evaluated by '/etc/rc.conf.local'

This 'hook' is installed automatically like this:

    # cat /etc/rc.conf.local
    ntpd_flags=""
    isakmpd_flags="-K -v"
    ipsec="YES"
    [ -f /etc/pkg_scripts.conf ] && for _r in `cat /etc/pkg_scripts.conf`; do pkg_scripts="${pkg_scripts} ${_r}"; done