Previously SLiM could only start on display :0.0. This change makes it
possible to run multiple instances of SLiM at the same time.
* gnu/packages/patches/slim-display.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/display-managers.scm (slim): Use it.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
