Add show_actual_foldernames preference and configuration option.

When enabled this setting shows actual folder names rather than
the localised verions for the four default folders:

$rcmail_config['default_folders'] =
  array('INBOX', 'Drafts', 'sent-mail', 'spam', 'Trash');
Change show_actual_foldernames to be show_real_foldernames.

Change requested by alec@alec.pl.
Merge pull request #77 from dpc22/realnames

Add show_real_foldernames preference and configuration option.