lib: de-duplicate assertions in bt_component_add_{input,output}_port

lib: de-duplicate assertions in bt_component_add_{input,output}_port

Move assertions from bt_component_add_input_port and
bt_component_add_output_port to add_port.  Add a parameter to indicate
to add_port if it's adding an input or output port.  This is only used
to select the correct strings for the "no duplicate name" pre-condition
check.  I think this de-duplication helps ensure both the input and
output paths are equivalent.

Change-Id: I217794719231251bf73d0347d6208af9fcaf6a0d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10037
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>