lttng_strerror(): accept positive and negative codes

lttng_strerror(): accept positive and negative codes

Some functions return positive `lttng_error_code` enumerator values
while other return negative values.

Make lttng_strerror() accept positive and negative codes. This improves
the API UX and will make the documentation less complex.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I359cef98ae32991d1a01b7f2cd484ece7bab2df9