Give the 0th domain's page daemon thread a consistent name.

Page daemon threads for other domains show up in ps(1) output as
"pagedaemon/domN", so let that be the case for domain 0 as well.

Submitted by: Kevin Bowling <kevin.bowling@kev009.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14518