gdi32: Move the whole DC restoring functionality into set_dc_state, and rename it to restore_dc_state.

Move other declarations that belong in ws2def.h from winsock2.h.

The pointers should be written as NDR 4-byte fields instead of a
platform-dependent size.

Don't run tests that results in an ERROR_INVALID_HANDLE exception being raised.

Check for the context handle function pointers being non-NULL to
prevent exceptions being raised on platforms without these functions.