Remove increments from backup repo based on size instead of age

[ericzolf]

Based on an idea expressed at https://savannah.nongnu.org/bugs/index.php?34650

The idea would be to remove old increments until a certain size is reached and/or a certain amount of space has been freed up on the file system where the backup repo resides, so that one is sure that the next backup will have enough space left to succeed.

It would be even nicer to do it based on the average/max size of an increment.

Not any time soon but a nice enough feature to be considered.