I was just wondering about this myself.
Turns out that in Slackware the services are controlled using their 'run command' scripts under /etc/rc.d/ (nostalgia intensifies!).
This should do the trick for snmpd, assuming it's installed -
/etc/rc.d/rc.snmpd restart
I appreciate this is an old post but thought I'd respond anyway as it may save someone else a little time in the future.
Regards,
Dave.