pg_upgrade should truncate/remove its logs before running

I have seen this numerous times but had not dug into it, until now.

If pg_upgrade fails and is re-run, it appends to its logfiles, which is
confusing since, if it fails again, it then looks like the original error
recurred and wasn't fixed. The "append" behavior dates back to 717f6d608.

I think it should either truncate the logfiles, or error early if any of the
files exist. Or it could put all its output files into a newly-created
subdirectory. Or this message could be output to the per-db logfiles, and not
just the static ones:
| "pg_upgrade run on %s".

For the per-db logfiels with OIDs in their name, changing open() from "append"
mode to truncate mode doesn't work, since they're written to in parallel.
They have to be removed/truncated in advance.

This is one possible fix. You can test its effect by deliberately breaking one
of the calls to exec_progs(), like this.

- "\"%s/pg_restore\" %s %s --exit-on-error --verbose "
+ "\"%s/pg_restore\" %s %s --exit-on-error --verboose "