* Correct buffer_bytes > INT_MAX on BSD/amd64.
The sysctl vfs.bufspace returns either an int or a long, depending on
the value. Large values of vfs.bufspace will result in error messages
like:
couldn't get meminfo: cannot allocate memory
This will detect the returned data type, and cast appropriately.
* Added explicit length checks per feedback.
* Flatten Value() to make it easier to read.
* Simplify per feedback.
* Fix style.
* Doc updates.
Instead of doing the whole metric exposition in a platform specific collector
implementation, this creates and updates the metrics in meminfo.go and
expected a platform specific implementation of getMemInfo on
*meminfoCollector.
This removes some error handling, which should be fine. If the calls
fail, we will get the zeroes, which is a safe enough fallback.
Additionally, if the first sysctl (page_size) succeeded it is unlikely
that other ones will fail.
