sys/_{cpu,domain}set.h: don't require sys/param.h

Add machine/param.h for MAXCPU and MAXMEMDOM in the _KERNEL case of
sys/_cpuset.h and sys/_domainset.h, making them more self contained.

This eliminates the need to include sys/param.h before sys/cpuset.h and
sys/domainset.h.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44466