Add bt_common_get_term_size()

Add bt_common_get_term_size()

Using ioctl() instead of the `COLUMNS` environment variable because
`COLUMNS` is in fact a shell variable which is not exported.

The function is not supported on Windows yet.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I84942830f0392ff1aca3e1aff54fffc534ddda05
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1621
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>