Don't save unfinished message sent by guest

When using Nightfox's login matrix I went to send an email to sysop and found that an email by a previous guest was still hanging around. This could be abused by two users wishing to exchange messages by way of logging in, leaving a message and then dropping the call and another one dialing in to pick it up.

Also, since guest is user 0000 then if it's using 0000.draft.mail.msg then two guests could possibly be creating the same draft message.

I propose that guest messages are checked and deleted before a new message is sent and that appending the node number to the filename will stop guests stomping on each others messages.