The string defined by user to fill in the circuit ID
suboption of option 82 in the user-define mode.
If the value of hpnicfDhcpSnoop2IfOpt82CIDMode is
NOT 'userDefine', it is set to a zero-length string
automatically.
Otherwise it must be set to a non-zero length string.
The SET operation of this object must be with the SET
operation of hpnicfDhcpSnoop2IfOpt82CIDMode.