0.6.12.15:

0.6.12.15:
	turned interrupt_install_low_level_handler() into
		undoably_install_low_level_interrupt_handler(), which
		uses atexit() to arrange for the handler to be removed
		at exit, for tidiness in general and specifically
		because without the the interaction between the GC
		and the exit()-time segfaults caused by memory
		corruption is making it tricky to debug the
		OpenBSD port