this is only used to close non-stdio files in derivation sandboxes. we
may as well encode that in its name, drop the unnecessary integer set,
and use close_range to deal with the actual closing of files. not only
is this clearer, it also makes sandbox setup on linux fast by 1ms each
Change-Id: Id90e259a49c7bc896189e76bfbbf6ef2c0bcd3b2
eldritch horrors
c7d97802e4