Not all extensions can be enabled or disabled at runtime, list the
extensions which can from the help message rather than on error only.
v2:
* Print the header message in the ListStaticExtensions() (Peter
Hutterer)
* Do not export ListStaticExtensions() as Xserver API
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>