NAT/Network: Add missing Id/@file/copyright headers.

scm automatic cleanups.

NAT/Net: Bump up TCP_SND_QUEUELEN. IPv6 has smaller MSS, so it needs more pbufs to send the same amount of data (reported as lwip bug 42737).

Provide LWIP_DEBUGF definition that uses VBox logging facilities. lwIP's default debugging level corresponds to VBox level4 so to get all the gory details of the stack operation (a lot of them) use VBOX_LOG="+lwip*.e.l2.l3.l4"

TCP_OVERSIZE_DBGCHECK is defined automatically for LWIP_DEBUG if TCP_OVERSIZE is set, so don't define it here.

LWIP_DEBUG is only tested for being defined. Don't define it as LWIP_DBG_ON as that creates wrong impression about the opposite choice.

LWIP_NOASSERT is tested with #ifdef, not #if. Drop it its incorrect definition from lwipopts.h and instead disable it in the Makefile for release, profile and kprofile builds.

#define ETH_PAD_SIZE 2 - to make IP header &c aligned.

Get rid of LWIP_SOCKET - we don't need it for NAT and it pollutes namespace.

Drop lwip-namespace.h renaming for VBoxNetNAT. It is only necessary in VBoxDD to avoid conflicts with slirp. #undef LWIP_PREFIX_BYTEORDER_FUNCS - its meaning was inverted in lwip-1.4 and lwip-namespace.h has to redo the same redefinition dance that lwip's def.h does when LWIP_PREFIX_BYTEORDER_FUNCS is undefined (this is to be addressed for VBoxDD separately). Debugging is more comfortable when names are not randomly renamed with some of them being original short names and some redefined with lwip_ prefix. Almost the same object code is generated, with slighly different offsets into .rodata because of changed __func__.

Give this file its own multiple inclusion protection guard.

Use component-specific lwipopts.h instead of sharing single one. This commit just replicates existing lwipopts.h and the resulting object code is identical.