vm_pageout: Add a chicken switch for multithreaded PQ_INACTIVE scanning

Right now we have the vm.pageout_cpus_per_thread tunable which controls
the number of threads to start up per CPU per NUMA domain, but after
booting, it's not possible to disable multi-threaded scanning.

There is at least one workload where this mechanism doesn't work well;
let's make it possible to disable it without a reboot, to simplify
troubleshooting.

Reviewed by: dougm, kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D48377