vl: Add DT_COCOA DisplayType value

Instead of reusing DT_SDL for Cocoa, use DT_COCOA to indicate
that a Cocoa display was requested.

configure already ensures CONFIG_COCOA and CONFIG_SDL are never
set at the same time. The only case where DT_SDL is used outside
a #ifdef CONFIG_SDL block is in the no_frame/alt_grab/ctrl_grab
check. That means the only user-visible change is that we will
start printing a warning if the SDL-specific options are used in
Cocoa mode. This is a bugfix, because no_frame/alt_grab/ctrl_grab
are not used by Cocoa code.

Cc: Andreas Färber <andreas.faerber@web.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
