The RASTER DDX does not validate the "default-printer-resolution" attribute.

Steps to reproduce:
1. Add
-- snip --
rastertest.xp-model-identifier: RASTERdefault
rastertest.xp-ddx-identifier: XP-RASTER
-- snip --
to /etc/X11/xserver/C/print/attributes/printer
2. Add
-- snip --
Printer rastertest
-- snip --
to /etc/X11/xserver/C/print/Xprinters
3. Set default resolution for all printers to "600" (in
/etc/X11/xserver/C/print/attributes/document):
-- snip --
*default-printer-resolution: 600
-- snip --

Result:
Wrong output by "xplsprinters":
-- snip --
 (XPSERVERLIST=:40 ./xplsprinters -printer rastertest -dump)     
printer: rastertest@:40
        description=
        model-identifier=RASTERdefault
        default-medium=na-letter
        default-input-tray=
        medium-source-sizes-supported=na-letter false 6.35 209.55 6.35 273.05
XpuFindResolutionByName() returned no match for default resolution '600'
        resolution=300dpi (300x300)
        default_orientation=portrait
        orientation=portrait
        orientation=landscape
        default_plex=
        plex=simplex
--> Job
*notification-profile:  {}
*qualifier:     rastertest

--> Doc
*copy-count:    1
*default-printer-resolution:    600
*qualifier:     rastertest
*content-orientation:   portrait
*default-medium:        na-letter

--> Page

--> Printer
*xp-raw-formats-supported:      {Postscript 2 } 
*document-formats-supported:    {Postscript 2 } 
*xp-page-attributes-supported:
*document-attributes-supported: default-medium document-format copy-count
*xp-setup-proviso:
*xp-model-identifier:   RASTERdefault
*job-attributes-supported:      job-name job-owner notification-profile
xp-spooler-command-options
*medium-source-sizes-supported: {'' {na-letter False {6.3500 209.5500 6.3500
273.0500}} } 
*plexes-supported:      simplex 
*input-trays-medium:
*printer-name:  rastertest
*xp-ddx-identifier:     XP-RASTER
*printer-resolutions-supported: 300 
*available-compressions-supported:      0123 0 
*xp-listfonts-modes-supported:  xp-list-glyph-fonts 
*content-orientations-supported:        portrait landscape 
*qualifier:     rastertest
*xp-embedded-formats-supported:

--> Server
*document-attributes-supported: copy-count
*job-attributes-supported:      job-name job-owner notification-profile
xp-spooler-command-options
*multiple-documents-supported:  False
*locale:        C

image resolution 0
-- snip --
(compare "*printer-resolutions-supported: 300" vs
"*default-printer-resolution:    600" ...)