vz: use CLONE_PARENT to fork a container

It allows to avoid a proxy process.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
vz: use CLONE_PARENT to enter into CT

Signed-off-by: Andrey Vagin <avagin@openvz.org>
util: move spawn_{wait,wake} functions

They will be used in the VZ driver too

Signed-off-by: Andrey Vagin <avagin@openvz.org>
vz: handle errors of execve

Signed-off-by: Andrey Vagin <avagin@openvz.org>
vz: support a new interface of child file descriptors

Signed-off-by: Andrey Vagin <avagin@openvz.org>
vz: set resource limits in one place

Signed-off-by: Andrey Vagin <avagin@openvz.org>