remap: don't add remaps for a dead pid more than once

Unless we seek and re-read the PB images, the only way I can see to do this is
to keep a list of the previously seen dead pids and check if a new remap is in
that list.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>