Change "ctlstat -P"'s schema

It now reports stats separately for both ports and luns.

Also, prohibit using both "-p" and "-P" at the same time.

Sponsored by: Axcient
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D38500
Approved by: re (cperciva)

(cherry picked from commit 4c163a5480809d0dc8b68dd00bf2ba7d882450f9)