main: really ignore SIGPIPE

[pkova]

When testing 410 with vere-v3.2 I've repeatedly ran into an issue where my ship turns off every few days. This was caused by the king receiving a SIGPIPE, something we ostensibly are ignoring with sigaction, however:

In a multi-threaded application, there is always the question of which thread the signal will actually 
be delivered to. Or does it get delivered to all the threads? [...]

If it is an asynchronous signal, it could go to any of the threads that haven't masked out 
that signal using sigprocmask().

Our "multi-threaded" application is the libuv thread pool. Using pthread_sigmask fixes the issue since all child threads inherit the signal mask.