tvheadend: do not send debug message to syslog

  Apparently tvheadend by default uses the syslog service, and the `-s`
  option that we provide in the startup script in fact makes tvheadend
  send debug messages to syslog (which we obviously do not want by default
  as this causes for my single DVB card 1k/min to /var, which is 4MB/month).
  So this fix disables the `-s` option in the tvheadend startup script.

  closes #2053, closes #2055