ptpd: systemd support

[Thomas:
   - use relative symbolic link instead of absolute one
   - remove the -S option, since redirecting stdout is not needed,
     it's already taken care of by systemd. Suggested by Maxime
     Hadjinlian.]

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