Export a list of VM objects in the system via a sysctl. The list can be
examined via 'vmstat -o'. It can be used to determine which files are
using physical pages of memory and how much each is using.

Differential Revision: https://reviews.freebsd.org/D2277
Reviewed by: alc, kib
MFC after: 2 weeks
Sponsored by: Norse Corp, Inc. (forward porting to HEAD/10)