pstree: introduce support for namespaces

Options -N and --ns-sort were added which require one of the namespaces:
ipc, mnt, net, pid, user, uts
and will show separated trees per namespace

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
pstree: introduce namespace transition information

This patch adds a new option (-S, --ns-change) that will show
when a namespace was changed compared to parent's.

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