userspace/snmpd: add new wrsDiskTable

New table with information about mounted partitions

Add new wrsDiskTable with objects:
--wrsDiskMountPath
--wrsDiskSize
--wrsDiskUsed
--wrsDiskFree
--wrsDiskUseRate
--wrsDiskFilesystem

Additionally:
-- update of MIB
-- update Makefile
-- update init.c

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>