Report an error for too-large VLAN tags, PPPoE session numbers, and Geneve VNIs.

Report an error for too-large VLAN tags, PPPoE session numbers, and Geneve VNIs.

Also, make those unsigned values, with a separate argument to the
corresponding gen_ routines to indicate whether we *have* a value to
test; this prevents values of 0xFFFFFFFF from being treated as
equivalent to "no value" just because (on 2's complement machines; we
make no effort to support other types) 32-bit -1 and 32-bit 0xFFFFFFFF
have the same bit pattern.