Tomasz Torcz authored Jan 09, 2015 and Nick Mathewson committed Jan 11, 2015

  There were following problems:
  - configure.ac wrongly checked for defined HAVE_SYSTEMD; this
    wasn't working, so the watchdog code was not compiled in.
    Replace library search with explicit version check
  - sd_notify() watchdog call was unsetting NOTIFY_SOCKET from env;
    this means only first "watchdog ping" was delivered, each
    subsequent one did not have socket to be sent to and systemd
    was killing service
  - after those fixes, enable Watchdog in systemd unit with one
    minute intervals