parser: allow ct eventmask set new,related

parser: allow ct eventmask set new,related

In case of bitmask types (tcp flags, ct eventmask) nft
allows to use comma operator to test multiple values, i.e.
tcp flags syn,ack   ct event new,destroy etc.

But currently nft fails to use this when used in a statement, i.e.
   ... ct eventmask set new,destroy
gives:
syntax error, unexpected comma

This allows makes this work, it is the same as
   ct eventmask set new|destroy

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