Move any remaining files generated by pg_upgrade into an internal subdir

Move any remaining files generated by pg_upgrade into an internal subdir

This change concerns a couple of .txt files (for internal state checks)
that were still written in the path where the binary is executed, and
not in the subdirectory located in the target cluster.  Like the other
.txt files doing already so (like loadable_libraries.txt), these are
saved in the base output directory.  Note that on failure, the logs
report the full path to the .txt file generated, so these are easy to
find.

Oversight in 38bfae3.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier, Justin Prysby
Discussion: https://postgr.es/m/181A6DA8-3B7F-4B71-82D5-363FF0146820@yesql.se
Backpatch-through: 15