Test: unix socket: test credential passing

Since the credential passing over UNIX sockets now makes use of the pid,
the compatiblity wrappers have become more complex as each platform
appears to define its own way of accessing this information.

This new test:
  - creates a named unix socket,
  - forks,
  - gets the parents and child to connect,
  - sends the child's credentials as a data payload and as credentials
    verified by the kernel
  - the parent checks that the two sets of credentials are equal.

This is more of a sanity check for the compatibility wrappers used on
non-Linux platforms.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ic0a6213afca7cc95a00617b052e7a145fc88625c