core,log: new 'MSG' log level between 'INFO' and 'WARN'

We're bumping the current "INFO" level messages to the new "MSG"
level, also making the new level the default.

The old "INFO" level will be used to setup an intermediate level of
logging which is not as verbose as "DEBUG" but still provides some
capabilities to analyze the behavior of a modem.

cli/mmcli-manager.c

@@ -70,7 +70,7 @@ static GOptionEntry entries[] = {
     },
     { "set-logging", 'G', 0, G_OPTION_ARG_STRING, &set_logging_str,
       "Set logging level in the ModemManager daemon",
-      "[ERR,WARN,INFO,DEBUG]",
+      "[ERR,WARN,MSG,INFO,DEBUG]",
     },
     { "list-modems", 'L', 0, G_OPTION_ARG_NONE, &list_modems_flag,
       "List available modems",