Fix: add missing cases in enum-to-string functions

Change the enum-to-string functions to not use a default case, such that
the compiler will give a -Wswitch warning if an enumerator is not
handled.  Add some missing ones found by such warnings.

Put a bt_common_abort call after each switch (rather than returning
"(unknown)", because there's no way in theory that we can reach that.
Having anything else would be a bug we would want to fix in babeltrace.

Change-Id: I2cfae112386aede4844062be51dba009a1e9a056
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/6693
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>