libct: add a type of a handle that represents processes

A process handle allow to wait a process and get its exit code.

Signed-off-by: Andrew Vagin <avagin@openvz.org>
test/Makefile: the "all" traget must be the first one

Signed-off-by: Andrey Vagin <avagin@openvz.org>
libct: local_process_create_desc() should not return NULL

It must return an error code

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

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