Suppress SIGPIPEs during writes to our piped process. On single-threaded
applications, this is as simple as ignoring the signal. But since this
is process-wide, on multi-threaded applications, we need to use some
cumbersome `pthread_sigmask` manipulation.

Thanks to https://www.doof.me.uk/2020/09/23/sigpipe-and-how-to-ignore-it/
and http://www.microhowto.info:80/howto/ignore_sigpipe_without_affecting_other_threads_in_a_process.html