Reclaim unused ithread priorities for user time-sharing threads.

Leave one band of ithread priorities available below PI_SOFT for
demoted ithreads but reclaim additional ithread priorities for use by
user time-sharing threads. This is an ABI change in that PZERO moves
up so old ps and top binaries will not format priorities correctly on
newer kernels, but that is a cosmetic rather than functional change.

Reviewed by: kib, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35647