packet-mgcp.[hc]: Various changes:

- Add/use extended value-string;
- Move proto_register...() and proto_reg_handoff...() to the end of the file;
- Add editor modelines and adjust whitespace as appropriate.

Change-Id: Iede67c418d9f2b4e263113bb14e77d6a56a16225
Reviewed-on: https://code.wireshark.org/review/4387
Reviewed-by: Bill Meier <wmeier@newsguy.com>