authPrompt: let PAM message wrap if longer than entry

authPrompt: let PAM message wrap if longer than entry

Right now the whole authPrompt spreads out if a PAM message
comes in that longer than the entry.

This commit changes it to wrap instead, by forcing the
auth prompt to be a fixed width (slightly bigger than
the entry width was sized to previously).

https://bugzilla.gnome.org/show_bug.cgi?id=705037