Avoid moving data directory in upgrade testing.

Windows sometimes gets upset if we rename a large directory and then try
to use the old name quickly, as seen in occasional buildfarm failures.
So we avoid that by building the old version in the intended
destination in the first place instead of renaming it, similar to the
change made for the same reason in commit b7f8465c.
Improve code documentation about "magnetic disk" storage manager.

The modern incarnation of md.c is by no means specific to magnetic disk
technology, but every so often we hear from someone who's misled by the
label.  Try to clarify that it will work for anything that supports
standard filesystem operations.  Per suggestion from Andrew Dunstan.
pg_upgrade:  don't copy/link files for invalid indexes

Now that pg_dump no longer dumps invalid indexes, per commit
683abc73dff549e94555d4020dae8d02f32ed78b, have pg_upgrade also skip
them.  Previously pg_upgrade threw an error if invalid indexes existed.

Backpatch to 9.2, 9.1, and 9.0 (where pg_upgrade was added to git)