Suricata

This reduces the footprint of files touched when adding a new app-layer. SNMP serves as a good example as its simply:

src/app-layer-parser.c:    SCRegisterSnmpParser();

And perhaps use a better name, like "generate-app-layer" or "create-app-layer".

To be discussed, but should there be a flag to create a plugin or a "built-in"?