build: fix list-old-dirs / check-old-dirs

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.

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

This is the regression since a8267ecc3df0a.

Reviewed by: emaste
Approved by: emaste
Sponsored by: Sippy Software, Inc.
Differential Revision: https://reviews.freebsd.org/D52153
MFC After: 3 days