[MAJOR] implemented client-side support for PF_UNIX sockets

[MAJOR] implemented client-side support for PF_UNIX sockets

A new file, proto_uxst.c, implements support of PF_UNIX sockets
of type SOCK_STREAM. It relies on generic stream_sock_read/write
and uses its own accept primitive which also tries to be generic.

Right now it only implements an echo service in sight of a general
support for start dumping via unix socket. The echo code is more
of a proof of concept than useful code.