debian: provide an upstart job

debian: provide an upstart job

lldpd daemonization seems to confuse upstart (I think this is because
it also forks the monitor process, otherwise the daemonization is
pretty classic). Therefore, when we detect we are run by
upstart (because of UPSTART_JOB variable), we raise the SIGSTOP signal
instead of daemonizing. Upstart notices it and will resume us and
consider we are ready. No more misleading fork.

Also, ignore SIGHUP which is used for reload.