This request returns a non-NULL pointer to a list of screen pointers if one or
more screens support the X Print Service Extension; otherwise it returns NULL.
XpQueryScreens determines if the X Print Service Extension is present, and if
so, which of all the screens on the X Server support the X Print Service
Extension. Unlike many other extensions, the X Print Service Extension may be
restricted to a subset of all available screens - for example, a single X Server
may be supporting video displays on some screens and printers on others.
The list of screen pointers can be freed by calling XFree.