proxy: best-effort clean up of unix domain sockets on exit
proxy: best-effort clean up of unix domain sockets on exit

If we get Control+C (SIGINT) or regular kill (SIGTERM) or successful
exit we unlink the Unix domain socket.

xapi-project/message-switch,djs55/xcp-idl,johnelse/xcp-idl,xapi-project/message-switch,Chunjie/xcp-idl,xapi-project/message-switch,euanh/xcp-idl