remap: fix dead pid remap of /proc/<pid>

It turns out we can't just test for /proc/<pid>, because the kernel appends
(deleted), since the directory is actually deleted (vs. something like
/proc/1/mountinfo, where the file still exists in the unlinked directory,
so there is no (deleted)). See comment for details.

v2: s/ret/is_dead in /proc/<pid>/xxx test, split tests to correctly test
    both cases

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