Fix port mapping rules precedence
This fix comprises of followings:
- Manged port mapping rules precedence for empty remote host or wildcard external port entry as per datamodel
- 0 for external port value has been treated as any port as per datamodel, earlier it was treated as port 0
- External port and external port end range separator changed from ':' to '-' as described in firewall UCI
- Fix the behavior of protocol parameter, earlier in get the default value shown TCP but in firewall rule was applied for both TCP and UDP