Custom file upload does not work properly on Windows systems due to *nix-centric code in CustomField.php

A custom file upload field, invoking CustomField.php, will not work due to the way that the directory structure is handled when renaming the file after upload. An explode on the "/" character is used; this should be replaced with a PHP file handling function for compatibility across different systems (specifically Windows).

I'm using Drupal. I do not know if the patch is applicable to other CMSs.

An error such as the following may be produced on a Windows system:

Warning: rename
(C:\inetpub\wwwroot\CiviCRM\sites\default\files\civicrm\custom\egcsv_2491da39e3c5e8a85c588de3aee16776.csv,
C:\inetpub\wwwroot\CiviCRM\sites\default\files\civicrm\custom\C:\inetpub\wwwroot\CiviCRM\sites\default\files\civicrm\custom\egcsv_2491da39e3c5e8a85c588de3aee16776.csv):
The filename, directory name, or volume label syntax is incorrect.
(code: 123) in CRM_Core_BAO_CustomField::formatCustomField() (line 1669 of C:\inetpub\wwwroot\CiviCRM\sites\all\modules\civicrm\CRM\Core\BAO\CustomField.php).

I have not tested the patch on a *nix system as yet, but it should not affect the functionality.