Add UCI section general which holds the uci parameter network defining on
which interface(s) the snmp agent is reachable for inbound snmp requests
in case the firewall zone does not allow INPUT traffic by default.

For the different zones to which the different interfaces belong firewall
procd input rules are created making the snmp agent reachable on udp port
161.

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