CLEANUP: fix inconsistency between fd->iocb, proto->accept and accept()

There's quite some inconsistency in the internal API. listener_accept()
which is the main accept() function returns void but is declared as int
in the include file. It's assigned to proto->accept() for all stream
protocols where an int is expected but the result is never checked (nor
is it documented by the way). This proto->accept() is in turn assigned
to fd->iocb() which is supposed to return an int composed of FD_WAIT_*
flags, but which is never checked either.

So let's fix all this mess :
  - nobody checks accept()'s return
  - nobody checks iocb()'s return
  - nobody sets a return value

=> let's mark all these functions void and keep the current ones intact.

Additionally we now include listener.h from listener.c to ensure we won't
silently hide this incoherency in the future.

Note that this patch could/should be backported to 1.6 and even 1.5 to
simplify debugging sessions.
diff --git a/include/proto/proto_udp.h b/include/proto/proto_udp.h
index c452a10..d1f0698 100644
--- a/include/proto/proto_udp.h
+++ b/include/proto/proto_udp.h
@@ -22,6 +22,6 @@
 #ifndef _PROTO_PROTO_UDP_H
 #define _PROTO_PROTO_UDP_H
 
-int dgram_fd_handler(int);
+void dgram_fd_handler(int);
 
 #endif // _PROTO_PROTO_UDP_H