pg_upgrade's bindir options could be optional

Just a thought: To make things a bit easier, the bindir options of
pg_upgrade (-b/-B, --old-bindir/--new-bindir) could be made optional, I
think.  The new bindir should normally be the one that pg_upgrade itself
is in.  And the old bindir could be found out by looking at the
postmaster.opts file in the old data directory.  At least by default,
this would make the pg_upgrade invocation a lot more compact; it would
just be: pg_upgrade -d oldir -D newdir.

Comments?