tarsnap: don't delete backup if it is the only one for a source

Check if more than 1 backups for a source (eg /home) exist.
If only 1 exist for the source don't delete it.
tarsnap: add more backup exclude directories
tarsnap: remove file logging + sending of mail after backup

We can use cron functionality to get the output of the script
after execution. There is no need to reimplement mail functionality in
the script.
tmux rename bat.sh to tmux-bat.sh