Display and sorting of item counts

Sometimes I care when there are folders with lots of files in; it breaks some filesystems etc.

Patches add:
 * 'c' to display item counts on the main browser
 * 'C' to sort by item counts
 * locale(7) aware formatting of numbers (i.e. so I get "1,000" (comma-separated) numbers).

Not entirely convinced about the choice of keys or by the sort code; feel free to change if you don't like it.