Merge branch 'jk/mailsplit-maildir-muttsort' into maint

Sort filenames read from the maildir/ in a way that is more likely
to sort messages in the order the writing MUA meant to, by sorting
numeric segment in numeric order and non-numeric segment in
alphabetical order.

* jk/mailsplit-maildir-muttsort:
  mailsplit: sort maildir filenames more cleverly