Modify "ls" to sort its data faster, using the new gnulib mpsort

Modify "ls" to sort its data faster, using the new gnulib mpsort
module rather than qsort.  This is particularly a win in
environments where strcoll is slow, since mpsort typically calls
strcoll less often than qsort does.
* bootstrap.conf (gnulib_modules): Add mpsort.
* src/ls.c: Include mpsort.h.
(sorted_file, sorted_file_alloc): New vars, for a new vector of
pointers to the file info, for speed.
(clear_files, extract_dirs_from_files, sort_files, print_current_files):
(print_many_per_line, print_horizontal, print_with_commas):
(calculate_columns): Set and use new vector.
(initialize_ordering_vector): New function.