struct lttng_channel: split protocol ABI from instrumentation ABI

Duplicate struct lttng_channel to introduce a new struct
lttng_ust_abi_channel_config meant to hold configuration data within the
memory area belonging to the ring buffer private data. Now the
application and consumer private pointer will be a separate pointer,
for which the memory will be owned by the caller.

This will allow us to modify the layout of struct lttng_channel without
breaking the UST communication protocol.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I96684aa6658f59dc3332da6034bdf9e3f921261f