support for named styles

[bartvde@…]

Currently Mapserver does not support named styles. All styles defined in the MAP
file are default styles in WMS terms, requested with &styles= (empty style name).

If it would be possible to define styles in Mapserver (not attached to a layer),
styles could be re-used for layers. There is already a bug for that (812). Using
wms metadata somebody could advertize which named styles he would want to
include per layer.

I was also thinking if this might solve a problem I have been running into with
Chameleon. When drawing a selection, Chameleon needs to request 2 images (2
GetMap requests which can't be combined), one image showing the original map,
and one using an SLD to show the selection in red on top of the original map.

If we had named styles, could this not be done in one single GetMap request
instead of two, combining a NamedStyle and a UserStyle? Or is this not allowed
by the SLD spec?