net-snmp: modify init script for SNMPv3

net-snmp: modify init script for SNMPv3

This commit adds function 'snmpd_snmpv3_add' to the init script
to support SNMPv3 config parsing.

The new uci config section has the following configuration parameters:

config v3
	option username 'John'
	option allow_write '0'
	option auth_type 'SHA|MD5'
	option auth_pass 'passphrase'
	option privacy_type 'AES|DES'
	option privacy_pass 'passphrase'
	option RestrictOID 'yes|no'
	option RestrictedOID '1.3.6.1.2.1.1.1'

This new section is only relevant if the snmp_version 'v1/v2c/v3' or 'v3'
is set in the uci section 'general'.

Signed-off-by: Christian Korber <[email protected]>
Signed-off-by: Florian Eckert <[email protected]>