d/postinst: fixup userid for older termproxy tasks

d/postinst: fixup userid for older termproxy tasks

At the time when we can fix this up the new (and possibly an old)
server daemon process is running, so use the flock CLI tool from
util-linux to ensure we do the same locking as the server and thus we
avoid a race condition.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>