In our usage of glib logging, every g_critical()/g_return*()/g_warning()
is considered a bug. They must not occur in a regular program run,
so that we can run NetworkManager with G_DEBUG=fatal-warnings.

On the other hand, all nm_log() statements (even with severity <error>)
are valid conditions that can occur at runtime.

As a consequence, when nm-logging uses glib as backend, we must not use
G_LOG_LEVEL_WARNING level. Otherwise

  $ /usr/sbin/NetworkManager --debug --g-fatal-warnings

will dump core on the message

  "<warn> glib-version: cannot handle SIGUSR1 and SIGUSR2 signals. Consider upgrading glib to 2.36.0 or newer"

Thereby, downgrade the glib level for "<info>" to G_LOG_LEVEL_INFO.