Fix <sys/_bitset.h> and <sys/_cpuset.h> to not require <sys/param.h>.
- Hardcode '8' instead of NBBY in _BITSET_BITS.
- Define a private version of 'howmany' for use in __bitset_words().
- While here, move a few more things out of _bitset.h and _cpuset.h to bitset.h and cpuset.h, respectively. The only things left in _bitset.h and _cpuset.h are the bits needed to define a bitset structure.
Reviewed by: bde, kib (ish)