lwip: enable SO_RCVBUF socket option

In issue #313 the SO_RCVBUF was intended to be enabled, however the current
lwip port looks for another define LWIP_SO_RCVBUF instead of LWIP_RCVBUF.

Fixes #716
netperf: use omni and rely on SO_RCVBUF available
lwip: use git and master branch
lwip: add "window scaling" support, RFC 1323

Apply patch from lwip webpage to recent lwip git master branch.