Improve directory validation in admin checks

This commit brings the following improvements in check_paths_inc.php:

• In addition to checking that a directory is valid, we now also
  verify that it is readable, this way admin will know if the error
  is caused e.g. by a symlink, or due to access rights
• Print actual path instead of a text description in install check.
  The config option name is already displayed as part of the first
  line of the check's output, so repeating the information does not
  add any additional value.
• Escape data printed in messages (path) with htmlspecialchars() as
  recommended by dhx
• Added comments to clearly identify purpose of each check block