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.

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