When constructing a pipe to child stdin on Posix, ignore SIGPIPE.

We can't count on every child process reading everything we try to write to
it. And if the child terminates with WritePipe data still pending, unless we
explicitly suppress it, Posix will hit us with SIGPIPE. That would terminate
the calling process, boom. "Ignoring" it means APR gets the correct errno,
passes it back to us, we log it, etc.