uipc: avoid circular pr_{slow,fast}timos

domain_init() gets reinvoked for each vnet on a system, so we must not
alter global state. Practically speaking, we were creating circular
lists and tying up a softclock thread into an infinite loop.

The breakage here was most easily observed by simply creating a jail
in a new vnet and watching the system suddenly become erratic.

Reported by: markj
Fixes: e0a17c3f063f ("uipc: create dedicated lists for fast ...")
Pointy hat: kevans