cmake: find liboath using the correct name

cmake: find liboath using the correct name

we should reference liboath by the $name in Find${name}.cmake, also the
$name should be consistent when calling find_package_handle_standard_args().
in this change
* rename Findliboath.cmake to FindOATH.cmake to be consistent with other
  find_package() moduless.
* use "OATH" in find_package_handle_standard_args() instead of "oath"
* set the interface properties for OATH::OATH, so the target linking
  against it can reference its header directories and libraries automatically.
* remove the stale comment for find_package_handle_standard_args()
* set OATH_INCLUDE_DIRS and OATH_LIBRARIES to follow the convention of
  find_package(), even they are not used directly in this project.

Reported-by: Erwan Velu <erwan@redhat.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>