GNU glibc 2.23 and later deprecate `readdir_r` in favour of plain old
`readdir` which in some upcoming POSIX standard is going to required to be
re-entrant.

Eventually we want to drop `readder_r` all together, but want to be
compatible with older unixen which may not have a re-entrant `readdir`.

Solution is to make systems with *known* re-entrant `readir` use that and
use `readdir_r` whereever we have it and don't *know* that `readdir` is
re-entrant.

Closes: https://github.com/haskell/unix/issues/70