Backport to 2.3:

Backport to 2.3:

Fixes for AF_UNIX support on OS/2:
- return the full size of the sockaddr_un structure, without which
  bind() fails with EINVAL;
- set test_socketserver to use a socket name that meets the form
  required by the underlying implementation;
- don't bother exercising the forking AF_UNIX tests on EMX - its
  fork() can't handle the stress.