json-lint.php: recursively lint json files with PHP

We need a way to lint JSON files using PHP json decoder which is
json_decode() PHP.  This new slave script accepts files and directories
as arguments, files are linted regardless of their extension,
directories are recursively traversed to find '.json' files.

Before PHP 5.5.0, we could only retrieve json error code, we thus have
to reimplement json_last_error_msg() to generate the same messages for
previous PHP versions.

Exit code of 1 is returned whenever a file fails.

Bug: 58279
Change-Id: I4e0d9383fa6cdf1b7e209b7b1964f8b2b2898aa9