Squash some more CLUSTER bugs. Never has worked on multiple-column

indexes, apparently, nor on functional indexes with more than one input
column (force of natts = 1 was in the wrong branch of IF statement).
Coredumped if source relation contained any uncommitted tuples, due to
failure to test for success return from heap_fetch.  Fetched tuple
was passed directly to heap_insert, which clobbers the TID and commit
status in the tuple header it's given, which meant that the source
relation's tuples all got trashed as the copy proceeded.  Abort partway
through, and you're left with a lot of missing tuples.
I wonder what else is lurking here ...