hostname-helper: use GLib transliteration API

Use GLib's new transliteration API to avoid the shortcomings of iconv().

Also solve an existing problem where strings entered in decomposed forms
wouldn't be properly transliterated.  Always normalise before attempting
transliteration.

Bump GLib dependency to 2.39.91 for the new feature.

https://bugzilla.gnome.org/show_bug.cgi?id=724194