When applying wireless configuration changes, the ifindex of the
wireless interface(s) change. While snmpd picks up the new interfaces
with the correct index, it does not remove the old ones:

IF-MIB::ifName.23 = STRING: wlan0
IF-MIB::ifName.24 = STRING: wlan1
IF-MIB::ifName.25 = STRING: wlan0
IF-MIB::ifName.26 = STRING: wlan1

This causes problems for monitoring tools that use ifName (or ifDesc) as
interface reference. Add a trigger that reloads snmpd on interface
up/down events so that it will no longer have the old interfaces.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>