GNU/Hurd branch

[gperciva]

We now have a branch which allows make test to pass on GNU/Hurd: https://github.com/Tarsnap/spiped/tree/DO-NOT-MERGE-hurd

This currently has two commits:

• GNU/Hurd does not support cancellation points:

  This is on their list of "open issues", although much of the info is from 2013 so I don't know how accurate it is:
  https://www.gnu.org/software/hurd/open_issues/libpthread_cancellation_points.html

• GNU/Hurd: shutdown(2) can produce EMIG_BAD_ID

  I think this is a kernel errno value, which their standard library should notice and replace with EBADF or ENOTSOCK.

These commits are not candidates for merging to spiped master.