rpc: prepare to receive packets with file descriptors

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
rpc: receive packets with file descriptors

It will be used to set file descriptors for executing processes.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
exec: add ability to set file descriptors for stdin, stdout, stderr (v2)

v2: add separate functions, which take file descriptor array

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>