Most of the sigactions are the same across the tasks in the image.
Nonetheless existing code always calls a syscall to restore them
and spends 64 calls per-task.

Let's restore signals before forking children and let them inherit
sigactions. Tune one only if it differs from the parent's.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>