lsns: print namespace tree based on the relationship (parent/child or owner/owned)

lsns: print namespace tree based on the relationship (parent/child or owner/owned)

Introduce new option -T[parent|owner]|--nstree[=parent|=owner].

With this change, lsns prints parent/child relationship tree if
"-T parent" is given and owner/owned relationship tree if "-T owner is given.

Passing only "-T" is same as passing "-Towner."

Example sessions:

    # ./lsns -Tparent -ons,type,pns| head -20
    NS             TYPE          PNS
    4026531837     user            0
    ├─4026532508   user   4026531837
    ├─4026532609   user   4026531837
    ├─4026532610   user   4026531837
    ├─4026532629   user   4026531837
    ├─4026532705   user   4026531837
    ├─4026532901   user   4026531837
    ├─4026533090   user   4026531837
    ├─4026533185   user   4026531837
    ├─4026533280   user   4026531837
    └─4026533468   user   4026531837
    4026531835     cgroup          0
    4026531836     pid             0
    └─4026533038   pid    4026531836
      ├─4026532934 pid    4026533038
      ├─4026533715 pid    4026533038
      ├─4026533716 pid    4026533038
      ...

    # ./lsns -Towner -ons,type,ons| head -20
    NS             TYPE          ONS
    4026531837     user            0
    ├─4026531835   cgroup 4026531837
    ├─4026531836   pid    4026531837
    ├─4026531838   uts    4026531837
    ├─4026531839   ipc    4026531837
    ├─4026531840   mnt    4026531837
    ├─4026531861   mnt    4026531837
    ├─4026532001   net    4026531837
    ├─4026532219   mnt    4026531837
    ├─4026532357   mnt    4026531837
    ├─4026532383   net    4026531837
    ├─4026532475   mnt    4026531837
    ├─4026532476   mnt    4026531837
    ├─4026532504   mnt    4026531837
    ├─4026532508   user   4026531837
    │ ├─4026532509 ipc    4026532508
    │ └─4026532511 net    4026532508
    ├─4026532573   mnt    4026531837
    ├─4026532574   mnt    4026531837

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