parser: use symbolic expressions for parsing keywords as protocol values

parser: use symbolic expressions for parsing keywords as protocol values

For "meta protocol" and the "meta nfproto" expressions, we need to be
able to parse "ip", "ip6", "vlan" and "arp" as protocol values.

Since the interpretation depends on the LHS of the relaltional expression,
we need to use symbolic expressions instead of constants to defer parsing
to the evaluation phase.

Signed-off-by: Patrick McHardy <kaber@trash.net>