prune: allow having all prune options zero/missing

prune: allow having all prune options zero/missing

This is basically necessary for the GUI's prune widget, because we want to
pass along all options equal to zero when all the number fields are cleared.
And it's more similar to how it's done in PBS now.

Bumped the APIAGE and APIVER, in case some external plugin needs to adapt to
the now less restrictive schema for 'prune-backups'.

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