restore daemon: create /run/proxmox-backup on startup

restore daemon: create /run/proxmox-backup on startup

fixes file restore again.

The new Memcom tracking file lives in `/run/proxmox-backup` and is
always created on REST interaction, as CachedUserInfo uses it to
efficiently track config changes, and such a cache is used in each
REST handle_request.

Further, the Memcom infra expects the base run PBS dir to exists
already, which is an OK assumption to have, but in the file-restore
daemon we have a significantly more minimal environment, and the run
dir was simply not required there, even /run isn't a tmpfs yet.

Fixes fda19dcc6f4 ("fix CachedUserInfo by using a shared memory version counter")
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>