"Admin => Misc" - Fix validation of "Maximum File Size"

"Admin => Misc" - Fix validation of "Maximum File Size"

The form "Administer => System Settings => Miscellaneous" has a field for "Maximum File Size".

I was on a workstation where the PHP-default and the Civi-default were disagreeable, so
(by default) I couldn't submit the form. So I noticed some quirks -- fixed below.

Before
------

* The error message tells you that the "Maximum File Size" disagrees with a setting in `php.ini`.
  Hopefully, you know what+where of "php.ini", otherwise you'll be at a loss for what value is acceptable.
* The validation assumes that `upload_max_filesize` is expressed in unit-megabytes. But `php.ini` actually
  allows different units (`2m`, `2048k`, `1g`, `2097152`). The comparison is incorrect
  if any other unit is used. (ex: If `php.ini has `upload_max_filesize=1g`, and if the requested
  limit is `2` megabytes, then it should accept - but it rejects due to mismatched units.)

After
-----

* Error message tells you what you need to know.
* Validator correctly interprets the units used in `php.ini`'s `upload_max_filesize`.