agent: clean up socket and port file on SIGTERM and SIGQUIT (#104)
agent: clean up socket and port file on SIGTERM and SIGQUIT (#104)

In case the agent is exiting due to a SIGTERM or SIGQUIT it is leaking
the port files in the gops config directory, as e.g. reported in
cilium/cilium#11455

Make sure the socket is cleaned up and port files in the gops config
directory are removed upon these signals by also relaying these signals
to the signal channel in gracefulShutdown. In case of SIGTERM, which is
the "normal" termination signal, exit the process with exit status 0.
google/gops