img: Move sigactions into core

Right now they all sit in a separate file. Since we
don't support CLONE_SIGHAND (and don't plan to) it's
much better to have them in core, all the more so
by the time we dump/restore sigacts, the core entry
is at hands already.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>