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.

contrib/postgres_fdw/deparse.c

@@ -734,7 +734,6 @@ build_tlist_to_deparse(RelOptInfo *foreignrel)
 	tlist = add_to_flat_tlist(tlist, foreignrel->reltarget.exprs);
 	tlist = add_to_flat_tlist(tlist,
 							  pull_var_clause((Node *) fpinfo->local_conds,
-											  PVC_REJECT_AGGREGATES,
 											  PVC_RECURSE_PLACEHOLDERS));
 
 	return tlist;