CRM-14092 - Restrict browsing of imageUploadDir via imageUploadURL

Previously, it attempted to restrict browsing of uploadDir and
configAndLogDir.  However, this is extraneoous because we have other checks
to ensure that those directories are inaccessible.  However, imageUploadDir
is different because we want to expose its file -- we just don't want to
expose a listing of them.

This commit also breaks out checkDirectoriesAreNotBrowseable() into
three functions.

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* CRM-14092:
  http://issues.civicrm.org/jira/browse/CRM-14092