Our pipe(2) system call internally creates data structures to enable
bi-directional traffic (inherited from FreeBSD). We don't currently
use this. We don't even expose it at all since the pipe descriptors are
marked read-only and write-only.

The question is: do we want to hang on to this?

If not, the code can be simplified, which, among other things, would
reduce complexity of an MP-safe locking design (something that FreeBSD
hasn't got right either, as far as I can tell).

-pk