Refactor pull_var_clause's API to make it less tedious to extend.

In commit 1d97c19a0f and later c1d9579dd8, we extended
pull_var_clause's API by adding enum-type arguments.  That's sort of a pain
to maintain, though, because it means every time we add a new behavior we
must touch every last one of the call sites, even if there's a reasonable
default behavior that most of them could use.  Let's switch over to using a
bitmask of flags, instead; that seems more maintainable and might save a
nanosecond or two as well.  This commit changes no behavior in itself,
though I'm going to follow it up with one that does add a new behavior.

In passing, remove flatten_tlist(), which has not been used since 9.1
and would otherwise need the same API changes.

Removing these enums means that optimizer/tlist.h no longer needs to
depend on optimizer/var.h.  Changing that caused a number of C files to
need addition of #include "optimizer/var.h" (probably we can thank old
runs of pgrminclude for that); but on balance it seems like a good change
anyway.

src/backend/optimizer/path/allpaths.c

@@ -2509,9 +2509,7 @@ qual_is_pushdown_safe(Query *subquery, Index rti, Node *qual,
 	 * Examine all Vars used in clause; since it's a restriction clause, all
 	 * such Vars must refer to subselect output columns.
 	 */
-	vars = pull_var_clause(qual,
-						   PVC_REJECT_AGGREGATES,
-						   PVC_INCLUDE_PLACEHOLDERS);
+	vars = pull_var_clause(qual, PVC_INCLUDE_PLACEHOLDERS);
 	foreach(vl, vars)
 	{
 		Var		   *var = (Var *) lfirst(vl);