restorer: isolate restorer like parasite (v4)

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>