[mini_snmpd] Support for dsk and memory

Mini SNMP is a tiny implementation of snmpd. It can run on embedded devices, like routers and storage devices. It is also part of the OpenWrt project.

Currently, this minimalistic daemon does implement only a very small subset information. When Obsersium tries to do discovery/polling these kind of devices, they cannot find any hrStorageEntries. This causes storage and memory pools undetected.

The information however is there, in example a dskEntry can be found when walking the information.

Currently I am working on a small implementation to support to load, memory and storage usage on those devices. After I finish, I'd like to contribute a patch when it is working.

However, as careful I as i will code, using snmp and observium is not my core business, so I might need someone to review and polish it a bit after it works

I already started working on it. When I have a fully working version, I will upload a patch against 0.13.10.4585 here.