remap: don't try to remap other files in /proc

We can't remap these files correctly anyway, so we should just return success
if we find one of these files to remap.

v2: don't try to remap accessible files in /proc

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
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>