ct: add packet/byte counter support

ct: add packet/byte counter support

packets and bytes need special treatment -- we want to be able to get
packet/byte counter in either direction, but also express
'fetch in *BOTH* directions', i.e.

ct packets original + ct packets reply > 1000

This either requires a '+' expression, a new 'both' direction, or
keys where direction is optional, i.e.

ct packets > 12345	; original + reply
ct original packets > 12345 ; original

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>