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.
