Merge parser's p_relnamespace and p_varnamespace lists into a single list.

Merge parser's p_relnamespace and p_varnamespace lists into a single list.

Now that we are storing structs in these lists, the distinction between
the two lists can be represented with a couple of extra flags while using
only a single list.  This simplifies the code and should save a little
bit of palloc traffic, since the majority of RTEs are represented in both
lists anyway.