server, server-core: Minimize fwd decs, use macro, and format

wayland-server.h: Adjust line breaks between prototypes.

wayland-server-core.h:
Adjust line breaks between prototypes.
Adjust space between splats and identifiers.
Remove unconventional linebreak before first parameter.
Add line breaks after return types.
Remove unnecessary forward declarations, and:
- move 'struct wl_client' declaration close to the dependent typedef
- tastefully move 'wl_shm_buffer_get' to leverage the return type
Replace explicit __attribute__ with WL_PRINTF macro.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>