Extend mail logging to allow mail to be logged AND send

We are hitting the situation where we need a copy of the mails going out for debugging purposes. When 'CIVICRM_MAIL_LOG' is defined the mail is logged but does not go out. We want the mail to be logged and still go out.

Dave suggested the CIVICRM_MAIL_LOG could be either TRUE of a value like 'both' - however, this constant already has handling for it to be set to either TRUE or a path.

At this stage I am simply defining another constant - 'CIVICRM_MAIL_LOG_AND SEND' - but I suspect that to go into core we'd need to look at making it a setting and presumably making that setting configurable via the UI - which takes it beyond what we want to put into it - so I will provide a link to our commit as a record of how to deal with that side of it & set this to 'future version'