debian: make serf runs under its own user/group
debian: update upstart script to also use _serf/_serf user
debian: use /etc/serf instead of /etc/serf.conf

Moreover, ship the configuration as an example.
debian: remove second upstart.conf in favor of the one in debian

Since only Ubuntu and derivatives are using upstart.conf, let's prefer
the one introduced in debian/ directory.
debian: use "respawn limit" to avoid fast restart of Serf for Upstart