Some tweaks to cut down on IDL stack usage. idl_intersection and idl_union

Some tweaks to cut down on IDL stack usage. idl_intersection and idl_union
now take only two arguments instead of 3, overwriting the result onto the
first argument. (glibc2.0.7 defaults to a 2MB stack per thread; 3 IDLs at
1.5MB plus various other runtime overhead is enough to trash the stack.)
Also pass in a tmp IDL from search_candidates instead of allocating it in
each candiate function.