Bug#221602: grub-installer: Should enable serial console on serial line installs

[Petter Reinholdtsen <pere@hungry.com>]

[Christian Perrier]
> Rewrite proposal according to general style used in d-i and DTSG (now
> linked on the Developer's Corner) aka Debconf Templates Style Guide.

Eh, none of the templates are intended for the user.  They are planned
to store automatically detected values.  The template text is just
documentation for the developers.  So I'm not sure if it is useful to
spend time making the questions look good in a dialog box. :)