This fixes a missing prototype warning in vl.c and obsoletes
the prototype in cocoa.m. Adjust callers in cocoa.m to supply
third argument, which is currently only used on Linux/ppc.

The prototype is designed so that it could be shared with SDL
and other frontends, if desired.

Cc: Alexandre Raymond <cerbere@gmail.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>