uma: multipage chicken switch

Add a switch to allow disabling multipage slabs, in order to facilitate
measuring memory usage and performance effects. The tunable
vm.debug.uma_multipage_slabs defaults to 1 and can be set to 0 to
disable. The name may change soon.

Reviewed by: markj (previous version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23487