Rewrote checksum_files.sh (and renamed it to checksum_file.sh), so that
less code is duplicated. Results, in LOC:

278 checksum_file.sh
231 checksum_file.sh

new file:   checksum_file.sh
deleted:    checksum_files.sh
Signed-off-by: Christian Kujau <github@nerdbynature.de>