WL#3422: 'mysql' client should show warnings on stderr in batch mode

See BUG#21099 for the motivation for this feature request.

My recommendation on this is to fix 5.1 so that:
- warnings are printed to stderr, not stdout, in batch mode; this will allow
them to be handled separately from the actual command output (currently
--show-warnings writes to stdout, mixed in with the normal output)
- warnings are printed by default in batch mode; --skip-show-warnings can be
specified if this is not desired