Implicit flow tables can't be listed

[Florian Westphal <fw@strlen.de>]

nft currently can't list implicit flow tables, e.g.:

 nft add rule ip filter forward flow { ip daddr counter }

... will work, and it shows up in

'nft list flow tables' as 'flow table __ft0' (or some other number) but

 nft list flow table filter __ft0
... will error out ('unexpected junk, expecting string').

We can either teach scanner.l that __ft{decstring} is a string,
or force users to always provide a name.

Any preference?

Thanks.