Since CL 3676, the TestMkdirAllAtSlash test
depends on syscall.EROFS, which isn't defined
on Plan 9.

This change works around this issue by
defining a system dependent isReadonlyError
function.

Change-Id: If972fd2fe4828ee3bcb8537ea7f4ba29f7a87619
Reviewed-on: https://go-review.googlesource.com/3696Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>