Bugfix: Rename proto_agent global channel variable

The PROTO_AGENT variable holding the channel for various was named as the same
one for FlexRAN. In C, global variables with the same name are merged into one,
which had the effect that the same queues where used for PROTO_AGENT and
FlexRAN, locking the FlexRAN RX thread. Renaming the PROTO_AGENT variable (it
was introduced second) resolves this issue.