distuninstallcheck: fail also when only one file is left installed

distuninstallcheck: fail also when only one file is left installed

This change fixes automake bug#9579.

* lib/am/distdir.am (distuninstallcheck): Be stricter in ignoring
a potential `dir' file created by install-info and left installed.
Also, be more careful about "this can't happen" kind of errors.
(am__distuninstallcheck_listfiles): New internal helper macro.
* tests/distcheck-pr9579.test: New test.
* tests/distcheck-override-infodir.test: Likewise.
* tests/Makefile.am (TESTS): Add them.
* NEWS, THANKS: Update.

Report by Nick Bowler.