gstat: Sync usage/synopsis + tag SPDX

Sort options using the slightly unusual convention used here as well.

MFC after: 3 days
Approved by: carlavilla (mentor), imp (src)
Differential Revision: https://reviews.freebsd.org/D48800

(cherry picked from commit 8b35839346d0bd8d4244018489bfdef8fd48de53)