CRM-15163 - CRM_Utils_Check_Security - Windows support

On Windows-PHP setups, paths may have a mix of back-slashes and
forward-slashes.  For this security check, we'll normalize to
forward-slashses to simplify the path manipulation.
Merge pull request #3975 from totten/4.4-win-check

CRM-15163 - CRM_Utils_Check_Security - Windows support