PBSClient: add file_restore_extract function

PBSClient: add file_restore_extract function

*_prepare creates a fifo for streaming data back to clients directly,
filefile_restore_extract blocks and should be called from a background
worker - while it is running outcoming data can be read from the FIFO.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>