Factor out open_backup_lockfile() method to acquire locks owned by
user backup with permission 0660.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
this is deprecated with rustc 1.52+, and will become a hard error at
some point:
https://github.com/rust-lang/rust/issues/79202
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>