[NET] Generalise tcp_listen_opt

This chunks out the accept_queue and tcp_listen_opt code and moves
them to net/core/request_sock.c and include/net/request_sock.h, to
make it useful for other transport protocols, DCCP being the first one
to use it.

Next patches will rename tcp_listen_opt to accept_sock and remove the
inline tcp functions that just call a reqsk_queue_ function.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/Makefile

@@ -2,7 +2,8 @@
 # Makefile for the Linux networking core.
 #
 
-obj-y := sock.o skbuff.o iovec.o datagram.o stream.o scm.o gen_stats.o gen_estimator.o
+obj-y := sock.o request_sock.o skbuff.o iovec.o datagram.o stream.o scm.o \
+	 gen_stats.o gen_estimator.o
 
 obj-$(CONFIG_SYSCTL) += sysctl_net_core.o