remove all old snapshots belonging to a job

remove all old snapshots belonging to a job

Changing maxsnap to something smaller can lead to left-over snaphsots otherwise,
as previously at most one snapshot would be removed, even if there are multiple
old snapshots according to the new setting.

Hopefully nobody relied on the fact that pve-zsync didn't clean up after itself
in such cases...

Negative values and 0 for 'maxsnap' should still be interpreted as infinity to
match the previous behavior.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>