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

NAT Network: Check if IP_MTU_DISCOVER defined instead of RT_OS_LINUX for consistency with other tests.

NAT Network: preserve IPv4 DF (is possible), TTL, TOS and IPv6 Hop Limit when proxying outbound UDP datagrams.

NAT/Net: convert perror/warn/warnx to DPRINTFs.

NAT/Net: Start untangling errno vs. winsock mess. Don't refer errno directly, while here convert some perror() calls to DPRINTFs.

NAT/Net: #define LOG_GROUP LOG_GROUP_NAT_SERVICE

Don't call pbuf_free() in proxy_sendto(), leave it up to the caller that may still need the pbuf, e.g. to generate an ICMP error.

Change vestigial names proxytest.* to proxy.*

NetworkServices: Whitespace (including tabs!) and svn:keywords cleanups by scm.

sys_mbox_tryfetch() returns timeout, not error code. Real tryfetch returns only 0 or SYS_MBOX_EMPTY, so this code worked ok in standalone mode since ERR_OK is also 0. In VBox tryfetch is faked with a fetch with timeout of 1, so successful fetch may return non-zero value, causing buggy caller to think tryfetch failed and leak memory.

Move proxy sources from Devices/Network/lwip-new/vbox to NetworkServices/NAT where they belong.