Populate more packet info fields

It is desirable to have the "protocol" and "info" fields of the packet_info properly populated. With these changes, then (for instance) tshark output looks like this:
```
 29   6.139663    127.0.0.1 60326 127.0.0.1    1883 MQTT CONNECT
 31   6.140923    127.0.0.1 1883 127.0.0.1    60326 MQTT CONNACK
 33   6.146678    127.0.0.1 60326 127.0.0.1    1883 MQTT PUBLISH
 35   6.147815    127.0.0.1 1883 127.0.0.1    60326 MQTT PUBREC
 37   6.148801    127.0.0.1 60326 127.0.0.1    1883 MQTT PUBREL
 39   6.150329    127.0.0.1 1883 127.0.0.1    60319 MQTT PUBLISH
 41   6.150631    127.0.0.1 1883 127.0.0.1    60326 MQTT PUBCOMP
 43   6.151260    127.0.0.1 60319 127.0.0.1    1883 MQTT PUBREC
 45   6.151664    127.0.0.1 1883 127.0.0.1    60319 MQTT PUBREL
 47   6.153478    127.0.0.1 60326 127.0.0.1    1883 MQTT DISCONNECT
 49   6.155721    127.0.0.1 60319 127.0.0.1    1883 MQTT PUBCOMP
```
Also, the display filter "mqtt" can be used.