Before this patch the restorer's code is linked in crtools and then
we copied functions from it. In this case all function should
be inline and we can't use a global variables.

I suggest to make it like parasite. The restorer's code is isolated in
own file and will be copied wholly.  The restorer's code is compiled as
position-independent code, so we can use functions and global variale
(E.g. to save descriptor for log messages).

v2: correct indentions in a separate patch
v3: introduce a variable restore_task_exec_start symmetrical to
    restore_thread_exec_start
v4: don't give command in restorer_thread()
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>