add SNMP readout support for DHT11 sensors

Add SNMP handlers for reading out humidity and temperature values
provided by DHT11 sensors.

Currently, ethersex is limited to only one such sensor. A future
extension to multiple sensors would be possible, the SNMP OID layout of
this implementation has been chosen with that possibility in mind.

SNMP OIDs are
SNMP_OID_ETHERSEX.5.1.n for the polling delay of sensor no. n [HZ]
SNMP_OID_ETHERSEX.5.2.n temperature sensor no. n [deg Celsius * 10]
SNMP_OID_ETHERSEX.5.3.n rel. humidity of sensor no. n [% * 10]

Only n=0 is implemented due to the constraints outlined above.