Add recbuf config option.

resolve #133 
Person who responsible for configuration of our services asked this, it is more convenient to tune sysctl's `net.ipv4.tcp_rmem`.

Regarded to doubling, when you do `gen_tcp:listen/2` without `recbuf` option, it does respect value from `net.ipv4.tcp_rmem` (does not doubling), although I don't know why.