iostat: getopt() / usage() cleanup.
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D41636
(cherry picked from commit 09c45b089d2f90ec04153a1b4621dd6bec142959)
Approved by: re (gjb)