Deprecate readdir_r.

I've been campaigning against readdir_r for years:

  http://elliotth.blogspot.com/2012/10/how-not-to-use-readdirr3.html

and glibc deprecated it in 2.24, so we should join the club.

Bug: http://b/32678321
Test: builds
Change-Id: Ie3887ed75989aced9324266aa3b93e85c11bdbc9