Some style and minor code improvements for idle selection.

Use designated initializers for the idlt_tlb elements.
Remove strstr() use, add flag field to detect supported MWAIT.
Use nitems() instead of the terminating NULL entry for idle_tlb.
Move several functions into cpu_idle_* namespace.

Based on the discussion with: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week