terminal-util: when prompting for a choice from a list, preselect longest prefix

terminal-util: when prompting for a choice from a list, preselect longest prefix

If all entries of a menu prompt start with the same prefix, let's
preselect the prefix to enhance user experience.

This is particularly relevant when prompting for a disk to install
things on, as typically they all start with the same prefix /dev/, and
if there's only a single target medium discoverable, then we can even
fill it out fully.