Use 'unbuild' instead of 'remove' in kernel prerm script

When DKMS is invoked by the kernel_prerm.d_dkms script to remove modules
from a kernel that is being removed, it uses the DKMS 'remove' command.
If there are no other installations of the module, it will be removed
completely from DKMS.  That seems to be an undesirable side-effect and
predates the introduction of the DKMS 'unbuild' command which merely
undoes the 'install' and 'build' steps.

Change the script to use 'unbuild' instead of 'remove'.  Keep the echoed
message that says "dkms: removing: ..." because "dkms: unbuilding: ..."
may be confusing to the user.

I have tested it by removing a kernel with a module installed by DKMS
that was the only installed instance of the module.  Without the change,
the module was removed completely from DKMS.  With the change, the
module was removed from the kernel, but was not removed completely.  On
reinstalling the kernel, the module was rebuilt and reinstalled for the
kernel by DKMS.

This change was suggested by @RalfGoebel for issue dell#37 and might be
Debian-specific.

Link: dell#37 (comment)
Tested-by: Ian Abbott <[email protected]>
Signed-off-by: Ian Abbott <[email protected]>

kernel_prerm.d_dkms

@@ -7,7 +7,7 @@ if command -v dkms > /dev/null; then
 	dkms status -k "$inst_kern" 2>/dev/null | while IFS=",:/ " read -r name vers _ arch status; do
 		[ "$status" = "installed" ] || continue
 		echo "dkms: removing: $name $vers ($inst_kern) ($arch)" >&2
-		dkms remove -m "$name" -v "$vers" -k "$inst_kern" -a "$arch"
+		dkms unbuild -m "$name" -v "$vers" -k "$inst_kern" -a "$arch"
 	done
 fi