coredump: Add new P and r variables

This adds two new variables to the coredump format.
 - P => global PID (PID in the init namespace)
 - r => root (root path for the process)

The main use for this is to make it easier to handle crashes that happened
within a container. With those two new variables it's possible to have the
crashes dumped into the container or forwarded to the host with the right PID
(from the host's point of view).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>