Fix Lintian warnings and errors.

W: mentat-ng: executable-not-elf-or-script etc/mentat/scripts/lib.sh
E: mentat-ng: systemd-service-file-outside-lib etc/systemd/system/mentat.service

lintian-info -t executable-not-elf-or-script
lintian-info -t systemd-service-file-outside-lib

(Redmine issue: #7365)