conf, log: On -h / --help, print usage to stdout, not stderr

Erik suggests that this makes it easier to grep for options, and with
--help we're anyway printing usage information as expected, not as
part of an error report.

While at it: on -h, we should exit with 0.

Reported-by: Erik Sjölund <erik.sjolund@gmail.com>
Link: https://bugs.passt.top/show_bug.cgi?id=52
Link: https://bugs.passt.top/show_bug.cgi?id=53
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

log.h

@@ -22,6 +22,7 @@ void debug(const char *format, ...);
 	} while (0)
 
 extern int log_trace;
+extern int log_to_stdout;
 void trace_init(int enable);
 #define trace(...)							\
 	do {								\