Fix ICE in "System.spawn".
Fix ICE in "System.spawn".

	* src/object/system.cc: Rename "system_class_spawn" into
	"system_spawn" and use the C++ binder to check the type
	of its argument.

aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi