snmp plugin: Improve subtree matching.

snmp plugin: Improve subtree matching.

Some MIBs use subtrees with an unusual OID naming schema. For example,
some network hardware encodes the MAC address in the last six parts of
the OID. Previously, the code only checked the last part of the OID
("sub-id") and assumed this was increasing. This assumption is not true
in such naming schemas and is not required by SNMP. This patch fixes
this behavior by comparing the entire "OID suffix".

This hopefully fixes Github issue #131.