Use start-stop-daemon to start/stop snmpd daemon.
Change name of init script from snmp to snmpd. Also rename link from S80snmp to
S80snmpd. Rename and update snmpd config for monit.

Distinguish between start-stop-daemon return value 0, 1 and the rest.
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>