CiviMail: Saner handling of Reply-To VERP-ing

As per discussions with Xavier et al., we should handle Reply-To VERP-ing in CiviMail in a saner way:

1. Move the 'Override VERP address?' checkbox from step 3 of the wizard to step 2.
2. Rename it to 'Track Replies?' and reverse its logic (i.e., use VERP-ed Reply-To if checked).
3. Reverse the default (as per <http://en.flossmanuals.net/CiviCRM/EveryDayMailManagement>) but make it configurable in civicrm.settings.php