Move direct call of ExitBootServices to efi_exit_boot_services. This
function sets boot_services_active to false so callers don't have to do
it everywhere (though currently only loader/bootinfo.c is affected).

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This starts to abstract whether or not boot services are available
Though maybe we should just set BS = NULL and let callers that care use that (or better: have all callers go through a wrapper that returns an error when it's gone)