parser: fix inconsistencies in set expression rules

parser: fix inconsistencies in set expression rules

Set keys are currently defined as a regular expr for pure sets and
map_lhs_expr for maps. map_lhs_expr is what can actually be used for
a single member, namely a concat_expr or a multiton_expr. The reason
why pure sets use expr for the key is to allow recursive set specifications,
which doesn't make sense for maps since every element needs a mapping.

However, the rule is too wide and also allows map expressions as a key,
which obviously doesn't make sense.

Rearrange the rules so we have:

set_lhs_expr:	concat or multiton
set_rhs_expr:	concat or verdict

and special case the recursive set specifications, as they deserve.

Besides making it a lot easier to understand what is actually supported,
this will be used by the following patch to support timeouts and comments
for keys in a uniform way.

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