Cursor: Refuse to free the root cursor

If a cursor reference count drops to 0, the cursor is freed.

The root cursor however is referenced with a specific global variable,
and when the root cursor is freed, the global variable may still point
to freed memory.

Make sure to prevent the rootCursor from being explicitly freed by a
client.

CVE-2025-26594, ZDI-CAN-25544

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

v2: Explicitly forbid XFreeCursor() on the root cursor (Peter Hutterer
<peter.hutterer@who-t.net>)
v3: Return BadCursor instead of BadValue (Michel Dänzer
<michel@daenzer.net>)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Part-of: <!1828>