20865: Don't use getentropy() on OSX Sierra.

Tor 0.2.9 has a broader range of fixes and workarounds here, but for
0.2.8, we're just going to maintain the existing behavior.

(The alternative would be to backport both
1eba0880 and
16fcbd21 , but the latter is kind of
a subtle kludge in the configure.ac script, and I'm not a fan of
backporting that kind of thing.)