Don't terminate on SIGPIPE

Zeus server receives SIGPIPE when a client quits unexpectedly (when it
gets killed, for any reason). This should not cause the server to
terminate; the go default SIGPIPE handler causes the server to react
correctly and remain alive and responsive (despite complaining about
the broken pipe).

I think this should take care of #239 and potentially @rosenfeld's "broken pipe" errors in #226.