machine-readable explain output v4

OK, here's the updated version of my machine-readable explain output
patch. This needed heavy updating as a result of the changes that Tom
asked me to make to the explain options patch, and the further changes
he made himself. In addition to any regressions I may have introduced
during the rebasing process, there is one definite problem here: in
the previous version of this patch, explain (format xml) returned XML
data; now, it's back to text.

The reason for this regression is that Tom asked me to change
ExplainStmt to just carry a list of nodes and to do all the parsing in
ExplainQuery. Unfortunately, the TupleDesc is constructed by
ExplainResultDesc() which can't trivially be changed to take an
ExplainState, because UtilityTupleDescriptor() also wants to call it.
We could possibly fix this by a hack similar to the one we already
added to GetCommandLogLevel(), but I haven't done that here.

...Robert