SITL: Avoid SIGPIPE on all platforms with a no-op SIGPIPE handler

This is a followup from diydrones/ardupilot#420

As suggested by @tridge, this commit uses a no-op `SIGPIPE` handler instead of `MSG_NOSIGNAL` for avoiding `SIGPIPE`. `MSG_NOSIGNAL` is Linux-only and thus does not work for avoiding `SIGPIPE` on OSX. This is a simpler, multi-platform solution.