Replace the static NGROUPS=NGROUPS_MAX+1=1024 with a dynamic
kern.ngroups+1.  kern.ngroups can range from NGROUPS_MAX=1023 to
somewhere in the neighborhood of INT_MAX/4 one a system with sufficent
RAM and memory bandwidth.  Given that the Windows group limit is
1024, this range should be sufficient for most applications

Only allocate the space we need before calling kern_getgroups instead
of allocating what ever the user asks for up to "ngroups_max + 1".  On
systems with large values of kern.ngroups this will be more efficient.

The now redundant check that the array is large enough in
kern_getgroups() is deliberate to allow this change to be merged to
stable/8 without breaking potential third party consumers of the API.