cocoa: Provide central qemu_main() prototype

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>

qemu-common.h

@@ -132,6 +132,11 @@ static inline char *realpath(const char *path, char *resolved_path)
 
 #endif /* !defined(NEED_CPU_H) */
 
+/* main function, renamed */
+#if defined(CONFIG_COCOA)
+int qemu_main(int argc, char **argv, char **envp);
+#endif
+
 /* bottom halves */
 typedef void QEMUBHFunc(void *opaque);