Ignore SIGPIPE in the main body of the dx function dispatcher

@psung @asimenos @jmdale 

This is a pretty minor/straightforward change to ignore SIGPIPE during the bulk of dx execution. SIGPIPEs on network sockets are translated into a different exception type, so they wouldn't be ignored here. The only caveat might be that I intend to catch SIGPIPE on stdout or stderr fds, but other fds might generate them as well (I don't know how to find which fd caused the signal).

Let me know if you can think of reasons not to do this.