Add support for systemd watchdog protocol

Systemd use a regular message on a socket to see if a process is alive,
and restart it if needed. In order to achieve that, a timer is added to
the main loop to notify systemd that we are still alive. This doesn't
add any overhead on a non-systemd enabled system, and requires to be
configured by the sysadmin, in the systemd unit file to be used.
See http://0pointer.de/blog/projects/watchdog.html for details.

This requires a version newer than 209, due to usage of sd_watchdog_enabled
function, and require to pass HAVE_SYSTEMD=1 to the Makefile.