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>