thread0: Stop calling thread0 "swapper"

Just leave it as "kernel". While here, replace "parked" with "-" (the
typical wait channel for idle threads).

Reviewed by: olce, kib, markj
Differential Revision: https://reviews.freebsd.org/D54039

(cherry picked from commit 431b3b224db2a69d09c359ce1cfb37ba3da4705c)