netsnmp: fix stop and restart actions in init script

/etc/init.d/S59snmpd stop (and restart) currently fail if either snmpd or
snmptrapd are not running.
This is because start-stop-daemon -K returns nonzero if the process to be
killed is not running, and this causes the whole script to fail because it runs
under 'set -e'.

This may not be noticed if only the last start-stop-daemon fails, but this is
in general wrong. It would also cause a failure in other scripts that execute
S59snmpd and check the return value.

Adding the --oknodo (-o) option to start-stop-daemon fixes this.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>