2nd take: dependency on libmime-perl is now on libmime-tools-perl
(not just build dependency)
Test the configuration file for existence rather than readability
This leads to a proper error message if later failing on it.
Replace line breaks with spaces in variables merged into header fields.
Line breaks are not supported inside header fields since \n is our field
separator.
Implement mailfiles_directory_flattened_folders to interpret folder names
as tag hierarchies on import.