payload: print base and raw values for unknown payloads

payload: print base and raw values for unknown payloads

We currently print 'unknown' rather than the raw offset values
for unrecognized header values.

If its unknown, prefer to print

payload @nh,0,16 set payload @nh,0,16

rather than 'unknown'.

Also add a helper to check if payload expression has a description
assigned to it.

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