osmux: Clean up mgcp_config osmux fields

* Move all of them to an anonymous "osmux" substruct to have them all
  together.
* Rename some fields to clarify its meanings, similar to what
  was already done in osmo-bts.
* Move the port VTY config next to the ip address ones.

Change-Id: Icc1c1ce3ab3a8beb941cda7c112485a0db882a90
7 files changed