ls(1): add a -v flag to sort naturally

Add a -v flag for ls which sorts entries following a natural ordering
using strverscmp(3) (e.g. "bloem1 bloem9 bloem10" as opposed to
"bloem1 bloem10 bloem9").

Update the manual page and add a test case.

Reviewed by: pauamma, bcr
Tested by: pstef
Differential Revision: https://reviews.freebsd.org/D36407