When displaying query results of type Maybe, one could use the
function rsMaybe. Unfortunately, this function maps 'Nothing'
values to RSNoData which gets displayed as '?' in the list of
query results. These semantics do not fit if the result is of
a Maybe type, because it is an optional field, because in this
case 'Nothing' is not an error (like for example the consequence
of an RPC error), but a legitimate result and as such should
be displayed as '-' in the result list. For this purpose,
this patch introduces another version of rsMaybe, which is
called rsMaybeUnavail to be used for optional fields.
For consistency, rsMaybe gets renamed to rsMaybeNoData.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>