fix(max_file_uploads): Changed handling of max_file_uploads for PHP >= 5.3.4 (refs #4544)

Starting with PHP 5.3.4, upload fields left blank on submission do not count
towards this limit.
fix(max_input_vars): Check for PHP's max_input_vars corruption (refs #4544)

PHP 5.3.9 introduced "max_input_vars" to set how many input variables may be
accepted.

We add a hidden input at the end of the form: if the variable is not available
in the received data, then it means the "max_input_vars" limit has been
reached.
fix(locale): Update message for max_input_vars and max_file_uploads (refs #4544)

Display user-friendly message for max_input_vars and max_file_uploads
cases.
Merge pull request #1087 from eguaj/3.3-A4544

fix(PHP): Prevent max_input_vars and max_file_uploads corruptions (closes #4544)