Refactor/optimize cpu_search_*().

Remove cpu_search_both(), unused for many years. Without it there is
less sense for the trick of compiling common cpu_search() into separate
cpu_search_lowest() and cpu_search_highest(), so split them completely,
making code more readable. While there, split iteration over children
groups and CPUs, complicating code for very small deduplication.

Stop passing cpuset_t arguments by value and avoid some manipulations.
Since MAXCPU bump from 64 to 256, what was a single register turned
into 32-byte memory array, requiring memory allocation and accesses.
Splitting struct cpu_search into parameter and result parts allows to
even more reduce stack usage, since the first can be passed through
on recursion.

Remove CPU_FFS() from the hot paths, precalculating first and last CPU
for each CPU group in advance during initialization. Again, it was
not a problem for 64 CPUs before, but for 256 FFS needs much more code.

With these changes on 80-thread system doing ~260K uncached ZFS reads
per second I observe ~30% reduction of time spent in cpu_search_*().

MFC after: 1 month