osmo-mgw: rename struct mgcp_trunk_config and symbol tcfg

rename struct mgcp_trunk_config to struct mgcp_trunk and the related
symbol name "tcfg" to "trunk" in order to better match the reality.

Change-Id: I02889dbf8149e139b1bd0326e13ce4c1aec867d1
Related: OS#2659
12 files changed