[MAJOR] make unix sockets work again with stats

[MAJOR] make unix sockets work again with stats

The unix protocol handler had not been updated during the last
stream_sock changes. This has been done now. There is still a
lot of duplicated code between session.c and proto_uxst.c due
to the way the session is handled. Session.c relies on the existence
of a frontend while it does not exist here.

It is easier to see the difference between the stats part (placed
in dumpstats.c) and the unix-stream part (in proto_uxst.c).

The hijacking function still needs to be dynamically set into the
response buffer, and some cleanup is still required, then all those
changes should be forward-ported to the HTTP part. Adding support
for new keywords should not cause trouble now.