ps(1), top(1): Priority: Let 0 be the first timesharing level

Change the origin from PZERO to PUSER.

Doing so allows users to immediately detect if some thread is running
under a high priority (kernel or realtime) or under a low one
(timesharing or idle).

MFC after: 1 month
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation