avtools: parse loglevel before all the other options.

This way it can be applied to cmdutils too -- e.g. showing the banner
and printing startup messages.

cmdutils.h

@@ -174,6 +174,11 @@ void parse_options(void *optctx, int argc, char **argv, const OptionDef *options
  */
 int parse_option(void *optctx, const char *opt, const char *arg, const OptionDef *options);
 
+/**
+ * Find the '-loglevel' option in the commandline args and apply it.
+ */
+void parse_loglevel(int argc, char **argv, const OptionDef *options);
+
 /**
  * Check if the given stream matches a stream specifier.
  *