AF_UNIX status report

[Ken Brown]

I've made at least rudimentary implementations of all the fhandler_socket_unix 
functions (including those in select.cc) for which there were previously only 
placeholders.

I've pushed everything to topic/af_unix, including a merge with master as of a 
couple days ago.

I've cobbled together a few test programs and put them in 
winsup/cygwin/socket_tests on the topic/af_unix branch.  I haven't taken the 
time to automate the tests, so they all have to be run interactively.  There is 
a Makefile to build the test programs and a README.txt that shows how to run them.

One thing I haven't yet done is to think about (or systematically test) datagram 
sockets.  I'm sure there's quite a bit of code that won't work for them.

Aside from datagram sockets, there are still a few things that I'm working on, 
but I'm close to the point where I could use some input:

1. I've littered the code in fhandler_socket_unix.cc and select.cc with FIXME 
comments on which I'd like advice.

2. I haven't given any thought at all as to how to implement SCM_RIGHTS 
ancillary data.  I could definitely use suggestions on that before I start 
thrashing around.

Ken