- reset errno before calling confstr - use confstr() doc to simplify
  checks afterwards
- Correct implementation and documentation of os.confstr.  Add a simple
  test case.  I've yet to figure out how to provoke a None return I can test.
- Address issues brought up by MvL on python-checkins.
  I tested this with valgrind on amd64.

  The man pages I found for diff architectures are inconsistent on this.
  I'm not entirely sure this change is correct for all architectures
  either.

  Perhaps we should just over-allocate and not worry about it?

The change to return None instead of "" in case of unconfigured
values has not been backported.