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>
pgrep: introduce support for namespaces

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