Fix list-old-dirs, check-old-dirs to not generate bogus "root fs" entries

[sobomax]

Summary

This is a regression since D34741, which causes bogus entry to be generated in some cases. It's quite a dangerous since if the output of the make command is fed to a dumb removal script it might kill the system.

Author: Maksym Sobolyev <[e-mail]>
Date: Mon Aug 25 10:16:41 2025 -0700

Fix list-old-dirs to not generate empty line at the end of the
output if OLD_DIRS happens to have a space at the end of the
last word of the output before the final \n. Then that space
is turned into \n and we end up with a blank line.

The make -V OLD_DIRS looks like so:

0000ce60  73 72 2f 74 65 73 74 73  2f 67 6e 75 2f 75 73 72  |sr/tests/gnu/usr|
0000ce70  2e 62 69 6e 20 75 73 72  2f 74 65 73 74 73 2f 67  |.bin usr/tests/g|
0000ce80  6e 75 2f 75 73 72 2e 62  69 6e 2f 64 69 66 66 20  |nu/usr.bin/diff |
0000ce90  75 73 72 2f 74 65 73 74  73 2f 67 6e 75 2f 6c 69  |usr/tests/gnu/li|
0000cea0  62 20 0a                                          |b .|
0000cea3

Futhermore this gets converted into a "/" i.e. root fs when
calling check-old-dirs.

$ make check-old-dirs
...
/lib/casper
/usr/lib/debug/lib/casper
/etc/skel
/etc/casper
/
/usr/lib/debug/
$

This is the regression since a8267ecc3df0a.

[emaste]

OK, but we should be able to just use :ts\n as in D39972.

[emaste]

(If you want to commit this one because you've already tested it I can pick up the :ts\n. Not sure why I missed -dirs when I did that for -libs and -files.

[sobomax]

Use :ts