First cut at doing something reasonable with OR-of-ANDs WHERE

First cut at doing something reasonable with OR-of-ANDs WHERE
conditions.  There are some pretty bogus heuristics in prepqual.c that
try to decide whether to output CNF or DNF format; they need to be replaced,
likely.  Right now the code is probably too willing to choose DNF form,
which might hurt performance in some cases that used to work OK.
But at least we have a foundation to build on.