r6088: Add the socket_wrapper library. This is a very simple library that

r6088: Add the socket_wrapper library. This is a very simple library that
redirects traffic (currently just IP traffic) over unix domain sockets
if the SOCKET_WRAPPER_DIR environment variable has been set.
Aim is to use this for the Samba4 torture suite on the buildfarm.

The socket_wrapper library can only be used if Samba was compiled with
--enable-developer.

test_rpc.sh passes against a local smbd with SOCKET_WRAPPER_DIR set.
(and ethereal showed no traffic whatsoever)

Stuff that still needs to be fixed in socketwrapper:
 - Give ENETUNREACH if target is not localhost
 - A given port number can only be used for UDP /or/ TCP, not both.
 - Perhaps allow some calls to circumvent socketwrapper (do we need DNS?)
(This used to be commit f8a63a843ccca092d9756b64e09175d37c08550a)