lsns: unify the code and option for printing process based tree and namespace based trees

Suggested by Karel Zak.

`--tree` option is introduced as replacement for `--nstree`.
In addition to `parent` and `owner`, `--tree` take `process` as the
option argument.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
2 files changed