[RFC] add SIOBKLGQ command to unix socket ioctl

Hi everyone, in the uWSGI project we use TCP_INFO
datas to know the status of the socket backlog queue (this helps
the master process identifying overloads)

http://projects.unbit.it/uwsgi/browser/master.c#L202

We would like to have a similar feature in unix sockets.
After a bit of investigation we decided to add an ioctl to get the backlog
size. This has been applied only to unix socket but i suppose it could be
used for other network families.

Patch follow

-- 
Roberto De Ioris
http://unbit.it