BUG/MEDIUM: cpuset: fix cpuset size for FreeBSD

BUG/MEDIUM: cpuset: fix cpuset size for FreeBSD

Fix the macro used to retrieve the max number of cpus on FreeBSD. The
MAXCPU is not properly defined in userspace and always set to 1 despite
the machine architecture. Replace it with CPU_SETSIZE.

See https://freebsd-hackers.freebsd.narkive.com/gw4BeLum/smp-in-machine-params-h#post6

Without this, the following config file is rejected on FreeBSD even if
the machine is SMP :
  global
    cpu-map 1-2 0-1

This must be backported up to 2.4.