[MAJOR] create proto_tcp and move initialization of proxy listeners

Proxy listeners were very special and not very easy to manipulate.
A proto_tcp file has been created with all that is required to
manage TCPv4/TCPv6 as raw protocols, and provide generic listeners.

The code of start_proxies() and maintain_proxies() now looks less
like spaghetti. Also, event_accept will need a serious lifting in
order to use more of the information provided by the listener.
diff --git a/Makefile b/Makefile
index 5fa8622..2d4ede2 100644
--- a/Makefile
+++ b/Makefile
@@ -231,7 +231,7 @@
        src/time.o src/fd.o src/regex.o src/cfgparse.o src/server.o \
        src/checks.o src/queue.o src/client.o src/proxy.o src/proto_uxst.o \
        src/proto_http.o src/stream_sock.o src/appsession.o src/backend.o \
-       src/senddata.o src/dumpstats.o \
+       src/senddata.o src/dumpstats.o src/proto_tcp.o \
        src/session.o src/hdr_idx.o src/ev_select.o src/acl.o src/memory.o
 
 haproxy: $(OBJS) $(OPT_OBJS)