remap: add a dead pid /proc remap

If a file like /proc/20/mountinfo is open, but 20 is a zombie (or doesn't exist
any more), we can't read this file at all, so a link remap won't work. Instead,
we add a new remap, called the dead process remap, which forks a TASK_HELPER as
that dead pid so that the restore task can open the new /proc/20/mountinfo
instead.

Previously, TASK_HELPERs died before the restore stage took place. However, the
fds are set up during the restore stage, so we need TASK_HELPERs to live at
least until then. This patch also rearranges some of the synchronization to
keep TASK_HELPERs alive until the restore stage ends.

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