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

Conflicts:
	CRM/Utils/Check/Security.php
CRM-14092 - CRM_Utils_Check_Security::isBrowseable - Fix warning

We don't care if there's an HTTP 404 when checking for browsability --
because 404 is another of saying "not browseable!"
CRM-14091, CRM-14092 - Defensive programming
CRM-14091, CRM-14092 - restrictAccess - Avoid unnecessary writes

Conflicts:
	CRM/Utils/File.php