* Add diskstats collector for OpenBSD
Tested on i386 and amd64, OpenBSD 6.4 and -current.
* Refactor diskstats collectors
This moves common descriptors from Linux, Darwin, OpenBSD
diskstats collectors into diskstats_common.go
Signed-off-by: Ralf Horstmann <ralf+github@ackstorm.de>
