Using MSG_NOSIGNAL on socket sends in libzookeeper_mt

[Stephen Tyree]

Right now when send is called after the cluster has closed the connection
(say on session expiration) SIGPIPE is fired for the entire application.
This seems like something we shouldn't be leaking out of the API, so a
simple fix (at least for Linux) would be to add the MSG_NOSIGNAL option to
any socket sends we do. This isn't portable to other platforms, but on other
platforms there are similar solutions to the problem. What does everyone
think?