os, syscall: revert Yosemite readdir workaround
Reverts https://golang.org/cl/
119530044 (OS X 10.10 Yosemite beta
14A299l workaround), since it was fixed in the final Yosemite release.
I verified that the C program http://swtch.com/~rsc/readdirbug.c
passes on Yosemite.
Adds a new test to the os package too, to verify that reading a
regular file as a directory fails.
Fixes #9789 (ReadDir: no error if dirname is a file)
Change-Id: I75286cef88fbb2ebccf045b479e33c810749dcbc
Reviewed-on: https://go-review.googlesource.com/4164
Reviewed-by: Dave Cheney <dave@cheney.net>