kernel - Refactor cpumask_t to extend cpus past 64, part 1/2

* 64-bit systems only.  32-bit builds use the macros but cannot be expanded
  past 32 cpus.

* Change cpumask_t from __uint64_t to a structure.  This commit implements
  one 64-bit sub-element (the next one will implement four for 256 cpus).

* Create a CPUMASK_*() macro API for non-atomic and atomic cpumask
  manipulation.  These macros generally take lvalues as arguments, allowing
  for a fairly optimal implementation.

* Change all C code operating on cpumask's to use the newly created CPUMASK_*()
  macro API.

* Compile-test 32 and 64-bit.  Run-test 64-bit.

* Adjust sbin/usched, usr.sbin/powerd.  usched currently needs more work.
kernel/netgraph7: Build fix.