ptpd2: systemd support

Following package and sysv initscript naming scheme,
the service is "ptpd2" instead of just "ptpd" version 2.

[Thomas:
  - use relative symlink instead of an absolute symlink
  - remove the -s option, which redirects stdout, since systemd will
    take care of it
  - add the -g option, to be consistent with what the init script is
    using.]

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>