scp_backup_to is not idempotent

[mgedmin]

If I do scp_backup_to /somewhere and run pov-simple-backup -s twice, I end up with two copies of everything:

/somewhere/2018-10-17/foo.tar.gz
/somewhere/2018-10-17/2018-10-17/foo.tar.gz

(If I run it a third time, I don't get a third copy, instead the 2nd copy is overwritten.)

Note that this interacts interestingly with #8: if BACKUP_SUFFIX is -mail I end up with

/somewhere/2018-10-17/foo.tar.gz
/somewhere/2018-10-17/2018-10-17-mail/foo.tar.gz