build: fix generation of ChangeLog

We must be able to have one when git is not present because we include
it in dist, but we also must generate it on each release. The previous
fix would keep the ChangeLog empty or not updated. We use a PHONY
target instead (and not a real PHONY target since we cannot add it
because automake doesn't like it).
redhat: update lldpd.spec
redhat: fix Redhat packaging

Call /sbin/ldconfig on postinstall.
debian: update Debian packaging.

The diff from the official package is now a lot smaller.
README: mention multi-OS support

And mention `lldpcli` in place of `lldpctl`.
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.
lldpd: remove uncleaned control socket when noone is listening
debian: add systemd support

Provides a lldpd.service file and let `lldpd` notify systemd when it
is ready using the notification socket.
priv: mirror the exit status of the child

When exiting because the child has exited, the privileged processus
will mirror the exit status and the signal that killed the child if
any. This allows monitoring daemon like systemd to detect an
unexpected exit.