grml-lock: drop gdialog usage and don't wrap text in zenity

gdialog is a compatibility wrapper around zenity and encourages
users to switch to usage of zenity, let's follow this recommendation.

Older zenity versions (like 3.14.0) had a different wrapping
behavior than more current versions (like 3.30.0). To avoid
small dialogs with only one or two words on each line (which
is ugly and close to unreadable) let's avoid wrapping text.

While at it fix some minor issues reported by shellcheck.

This work was funded by Grml-Forensic.