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).
Merge pull request #266 from antifuchs/dont-exit-on-SIGPIPE

Don't terminate on SIGPIPE