lttng_statedump_process_state for each PID NS

When a process is in a namespace, its pid, tid and ppid are relative to
the namespace. Since namespaces can be nested, we need to know the
representation of each process in each namespace.

This patch changes the lttng_enumerate_task_fd to iterate over each
PID namespace of a process if any, that way we generate, in the
statedump, an entry for each process in each namespace it belongs.
To know the nesting level, the field "level" is added to the
lttng_statedump_process_state event, 0 being the top-level.
For processes running on the top-level namespace, the statedump
behaviour is unchanged (except the added "level" field).

For example (no nesting, just one level of namespace) :
lttng_statedump_process_state: {
        tid = 32185, vtid = 1, pid = 32185,
        vpid = 1, ppid = 32173, vppid = 0,
        level = 1, name = "init" }
lttng_statedump_process_state: {
        tid = 32185, vtid = 32185, pid = 32185,
        vpid = 32185, ppid = 32173, vppid = 32173,
        level = 0, name = "init" }

Confirmed that the process 32173 in the top-level namespace is indeed
the lxc-start command that created the container and its namespace.

[ Edit by Mathieu Desnoyers: Minor cleanups ]

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>