Send instrument target data through ustcomm

Define a function ustctl_set_target() that sends the instrument
target through ustcomm with the LTTNG_UST_TARGET command.
Add handler for LTTNG_UST_TARGET in UST ABI

Define a lttng_ust_target_node struct to store the target data.

Add message handler for the LTTNG_UST_TARGET command in
UST ABI. Copy the target data into a lttng_ust_target_node
structure and pass it to the corresponding enabler.