(file) Re-examine Path.checkAccess

The checkAccess method is currently very awkward to use, requiring applications to catch an exception for the case that access is denied, or some other I/O error occurs. Some developers have suggested methods equivalent to File's canXXX methods but they are problematic too because checking access can fail for many reasons.

In addition to re-examining this method, the spec for the exists/notExists method should be changed to not require the implementation use checkAccess.