compel: Initial commit for standalone tool

The compel component is a replacement for several aspects of CRIU
functionality: binary blobs generation for PIE parasite/restore code,
and a library for parasite code injection and execution (to be implemented).

In the commit we rather shuffle compel into own directory and
use it for

1) Fetching cflags when compiling PIE blobs
2) Use its "piegen" functionality to generate blobs themselves.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>