To later be properly implemented using winstation atom table.
The idea is to try to share the atom table in shared memory, to avoid round trips when reading window class info. The global atom table lives in ntdll and moving the session shared memory there seems more inconvenient than using atom tables in winstation shared objects, which could stay in win32u.
Note that there doesn't seem to be any dedicated syscall for RegisterClipboardFormat but it seem to be basically doing the same as RegisterWindowMessage, so I'm using the same syscall.
-- v2: user32: Implement RegisterClipboardFormat(A|W) using NtUserRegisterWindowMessage. user32: Implement RegisterWindowMessage(A|W) using NtUserRegisterWindowMessage. win32u/tests: Test some pinned global and user atoms. win32u/tests: Test that user atoms are not a global atoms. win32u: Implement NtUserRegisterWindowMessage.