(sort_type): Rearrange to use as an array index when

(sort_type): Rearrange to use as an array index when
choosing sort function; added new sort_numtypes member for
compile-time check.
(time_type): Add new time_numtypes member for compile-time check.
(directories_first): New global variable.
(GROUP_DIRECTORIES_FIRST_OPTION): New enum.
(long_options): Add --directories-first.
(main): Support new option.
(is_directory): New function.
(extract_dirs_from_files): Use it.
(DIRFIRST_CHECK, DEFINE_SORT_FUNCTIONS)
(LIST_SORTFUNCTION_VARIANTS): New macros.
(sort_functions): New global variable.
(sort_files): Use it.
(usage): Document new option.