Reworking vmmeter

[Benno Rice]

So I've put together this patch:

http://people.freebsd.org/~benno/vmmeter.diff

This patch does a few things:

- Renames the singleton "cnt" to "vmmeter".
- Replaces all the per-cpu counters with counter_u64_t.
- Removes the vmmeter instance from struct pcpu, due to the above mentioned change.
- Adds includes for vmmeter.h to a few files that were only getting it via pollution in pcpu.h
- Removes some entries from assym that weren't being used.

This has been tested on amd64 and nothing else right now, I'm more posting this to get general comments on whether people think this is a good idea. My motivation for this was twofold, firstly to rename cnt and secondly to move the counters to the common counter framework. More testing will be done prior to commit.

Cheers,
	Benno.