netfilter: nf_osf: add nf_osf_find()

This new function returns the OS genre as a string. Plan is to use to
from the new nft_osf extension.

Note that this doesn't yet support ttl options, but it could be easily
extended to do so.

Tested-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>