MFC r250395 (by attilio / jeff):
Generalize the bitset operations, present in cpuset and offer a KPI to
redefine such operations for different consumers.
This will be used when NUMA support will be finished and numaset
will need to be used.

(cherry picked from commit 537bb0ff8634622e03a53e2b023cfe684755df84)
MFC r251703 (by attilio):
 - Add a BIT_FFS() macro and use it to replace cpusetffs_obj()

(cherry picked from commit d7a5bc79fa57f61a00c8682255f7f3b0a0a52364)
This compensates MFC r251703 as we have an early HEAD merge which is
never part of stable/9.