parser_bison: Avoid set references in odd places

parser_bison: Avoid set references in odd places

With set references being recognized by symbol_expr and that being part
of primary_expr as well as primary_rhs_expr, they could basically occur
anywhere while in fact they are allowed only in quite a few spots.

Untangle things a bit by introducing set_ref_expr and adding that only
in places where it is needed to pass testsuites.

Make sure users can define variables as set references, eg.

	define xyz = @setref

And allow to use them from set expressions and statements too.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>