cpuset: increase userland maximum size to 1024

Hardware with more than 256 CPU cores is now available and will become
increasingly common. Bump CPU_MAXSIZE (used for userland cpuset_t
sizing) to 1024 to define the ABI for FreeBSD 14.

This change is reapplied after a change to decouple cpuset from bhyve:
commit e17eca327633 ("vmm: Avoid embedding cpuset_t ioctl ABIs").

PR: 269572, 271213 [exp-run]
Reviewed by: mjg, jhb
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39941

(cherry picked from commit 76887e84be975698b14699d7d0dfb157d73e9990)