net-snmp: fix inbound firewall rule support

net-snmp: fix inbound firewall rule support

Commit ae5ee6ba6c506b42d942c98349b3a54181790ec8 added support for inbound
firewall rule support but some corner cases were not covered.

In case net-snmp is started and the network interface is already up
the procd firewall rule is created but not applied by fw3 as
service_started calling procd_set_config_changed firewall was missing.

When stopping net-snmp clean up the net-snmp inbound firewall rules in
iptables by calling procd_set_config_changed firewall in stop_service
which will trigger fw3 to remove the inbound firewall rules.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>