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

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

NAT/Net: Get rid of inet_ntop(3) (not available on Windows XP). Since DPRINTFs now use Log*() calls we can use %RTnaipv4/%RTnaipv6.

NAT/Net: #define LOG_GROUP LOG_GROUP_NAT_SERVICE

Few more missed -1 -> INVALID_SOCKET replacements.

G/c leftover #if0'ed static port-forwarding used for testing.

Change vestigial names proxytest.* to proxy.*

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.