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>
it's not really needed in the config module, and this makes it easier to
disable the proxmox-openid dependency linkage as a stop-gap measure.
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>