Extend the VMM stats interface to support a dynamic count of statistics.

• Add a starting index to 'struct vmstats' and change the VM_STATS ioctl to fetch the 64 stats starting at that index. A compat shim for <= 13 continues to fetch only the first 64 stats.

• Extend vm_get_stats() in libvmmapi to use a loop and a static thread local buffer which grows to hold the stats needed.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D27463