lib: use `LIBBABELTRACE2_INIT_LOG_LEVEL` env. var. to initialize log level

lib: use `LIBBABELTRACE2_INIT_LOG_LEVEL` env. var. to initialize log level

Use the `LIBBABELTRACE2_INIT_LOG_LEVEL` environment variable instead of
`BABELTRACE_LOGGING_GLOBAL_LEVEL` to make it very clear that it's about
the library, and not related to the individual component log levels, for
example.

`INIT` is in the name to indicate that it's only the initial log level;
the library user can still change it at run time with
bt_logging_set_global_level(), while the other log level environment
variables, `BABELTRACE_PLUGIN_CTF_METADATA_LOG_LEVEL` and
`BABELTRACE_PYTHON_BT2_LOG_LEVEL`, are used to set the definitive log
levels of those modules.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ibf580bed6c30d88b4fbe3aed20f38cc969bc9295
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1511
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>