Improve admin checks for language strings

This brings the following improvements to test_lang.php:

• Simplify the code by defining a new checkplugins() function for
  plugins language checks, allowing to use a revised checklangdir()
  for both the main and the plugin language strings (removes the
  recursive call)

• Use PHP scandir() function to retrieve a sorted array of files
  instead of manually building it using opendir() and a loop on
  readdir()

• Process the languages in alphabetical order

• Array having strings as keys are valid tokens. Fixes 0013643
  This avoids reporting errors is useful for plugin error strings like
  $MANTIS_ERROR['plugin_example_error1'] = 'Error message';

• Better formatting of error messages

• Whitespace and coding guidelines fixes

NOTE: This commit has been ported to master for consistency, although
the new format of the strings_english.txt file is not compatible with
the logic in test_lang.php.