Improve backup mode management

This commit makes mandatory the presence of a full backup when doing
an incremental or archive backup on an existing timeline. In this case
the process will now simply error out and not take any backup. It looks
safer to use that as a default by the way, so as user will be forced
to take a full backup once a recovery has been done.

Database backup also contained the following condition when doing an
incremental backup:
prev_backup->tli != current.tli
This means that an incremental backup cannot be taken if there is not
already a full backup present in the same timeline. The same condition
should also be used for archive backup but it didn't seem to be the
case...
Fix typo