replace File::Pid dependency with Proc::PID::File

File::Pid is broken for perl > 5.12 and author has not patched in over
one year.  Fixes #4
fix freebsd init script for FreeBSD 10+

set_rcvar has been removed in FreeBSD 10
v0.52

    - add systemd init script for fedora/RHEL7 [Javier Wilson]
    - add 'reply' global configuration to change the milter response from
      SMFIS_REJECT to SMFIS_TEMPFAIL if desired
    - replace broken File::Pid dependency with Proc::PID::File
    - fix freebsd init script for FreeBSD 10