procps: add support for linux namespaces

Each process in Linux has a /proc/<pid>/ns directory which contains
symbolic links to pipes that identify which namespaces that process
belongs to. This patch adds support for ps to display that information
optionally.

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Aristeu Rozanski <aris@ruivo.org>
pgrep: introduce support for namespaces

A PID should be specified with --ns:
$ pgrep --ns 12345
which will only match the processes which belong to to the same 6
namespaces. It is also possible to specify which namespaces to test:
$ pgrep --ns 12345 --nslist mnt,net,ipc
which will match processes that belong to the same mount, network and
IPC namespaces as PID 12345.

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Aristeu Rozanski <aris@ruivo.org>
skill: support namespaces

In the same fashion of pgrep, introduce two new options:
--ns <pid>
- nslist <ns,...>
which allows processes to be filtered by namespace.

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Aristeu Rozanski <aris@ruivo.org>