Changed autoloader-ignore mechanism (refs #4186)

- Exclusion globs are now specified in files located in the
  `$wpub/.autoloader-ignore.d` directory.
- `.autoloader-ignore` files are not interpreted anymore, and they
  must be migrated to the `.autoloader-ignore.d` format.
Added `programs/check_autoloader` (refs #4186)

The `programs/check_autoloader` force a regeneration of the autoloader without
altering the actual autoloader cache: this allows to check and report
duplicated PHP classes.
Updated/added `CORE_premigr_3.2.{5,8,10}` scripts to delete `po2js.php` files (refs #4186)
Merge pull request #964 from eguaj/3.2-A4186

Changed autoloader-ignore mechanism (closes #4186)