#1397 Avoid duplicated registrations

[Gao Yahu]

The children nodes of a subtree always have the same OID with their first node.
While reattach all our nodes, the children nodes are not necessary to be reattached cause the first node always be registered before them.

This can fix the warnings:
...
duplicate registration: MIB modules AgentX ... and ...(oid .1.3.6.1.2.1.4).
duplicate registration: MIB modules AgentX ... and ...(oid .1.3.6.1.2.1.4).
duplicate registration: MIB modules AgentX ... and ...(oid .1.3.6.1.2.1.4).
duplicate registration: MIB modules AgentX ... and ...(oid .1.3.6.1.2.1.5).
...

Command:
sudo ./snmpd -Loe -C -X -f -I interface -I ip -I at -I icmp -I tcp -I udp -Dall > tmp.txt 2>&1

My platform like that:
% uname -a
Linux pek-ygao-d1 4.4.0-150-generic #176-Ubuntu SMP Wed May 29 18:56:26 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
% ./snmpd --version

NET-SNMP version: 5.8
Web: http://www.net-snmp.org/
Email: net-snmp-coders@lists.sourceforge.net