Add --task argument in lsns

[vbmade2000]

Closes #608

This PR adds a --task <pid> argument to show namespaces associated with supplied PID.

Option added to the CLI

vbmade2000@vbmade2000:~/util-linux/target/release$ ./lsns --help
List the namespaces in the system.

Usage: ./lsns [OPTION]...

Options:
  -n, --noheadings   don't print headings
  -P, --persistent   namespaces without processes
  -t, --type <name>  namespace type (mnt, net, ipc, user, pid, uts, cgroup, time)
  -p, --task <pid>   print process namespaces
  -h, --help         Print help
  -V, --version      Print version

Namespaces for the specific PID is shown with -p option.

vbmade2000@vbmade2000:~/Documents/projects/oss/util-linux/target/release$ ./lsns
        NS TYPE   NPROCS   PID USER       COMMAND
4026531834 time      106  2592 vbmade2000 /usr/bin/pipewire
4026531835 cgroup    106  2592 vbmade2000 /usr/bin/pipewire
4026531836 pid       105  2592 vbmade2000 /usr/bin/pipewire
4026531837 user      106  2592 vbmade2000 /usr/bin/pipewire
4026531838 uts       106  2592 vbmade2000 /usr/bin/pipewire
4026531839 ipc       106  2592 vbmade2000 /usr/bin/pipewire
4026531840 net       105  2592 vbmade2000 /usr/bin/pipewire
4026531841 mnt       103  2592 vbmade2000 /usr/bin/pipewire
4026532636 mnt         2  3719 vbmade2000 /snap/snapd-desktop-integration/391/usr/bin/user-session-helper /snap/snapd-desktop-integ
4026532637 mnt         0       root       
4026532638 mnt         1  9482 vbmade2000 /snap/firmware-updater/226/bin/firmware-notifier
4026532639 pid         1  4118 vbmade2000 /opt/zoho.com/ulaa/ulaa --type=zygote --crashpad-handler-pid=4110 --enable-crash-reporter
4026532640 net         1  4118 vbmade2000 /opt/zoho.com/ulaa/ulaa --type=zygote --crashpad-handler-pid=4110 --enable-crash-reporter
4026532820 mnt         0       root       
4026532879 mnt         0       root       
vbmade2000@vbmade2000:~/util-linux/target/release$ ./lsns -p 2592
        NS TYPE   NPROCS   PID USER       COMMAND
4026531834 time      106  2592 vbmade2000 /usr/bin/pipewire
4026531835 cgroup    106  2592 vbmade2000 /usr/bin/pipewire
4026531836 pid       105  2592 vbmade2000 /usr/bin/pipewire
4026531837 user      106  2592 vbmade2000 /usr/bin/pipewire
4026531838 uts       106  2592 vbmade2000 /usr/bin/pipewire
4026531839 ipc       106  2592 vbmade2000 /usr/bin/pipewire
4026531840 net       105  2592 vbmade2000 /usr/bin/pipewire
4026531841 mnt       103  2592 vbmade2000 /usr/bin/pipewire
myuser@mysystem:~/util-linux/target/release

Invalid PID check

vbmade2000@vbmade2000:~/util-linux/target/release$ ./lsns -p invalid_pid
./lsns: invalid PID argument: 'invalid_pid'

Command shows regular output (all namespaces) when using PID 0

vbmade2000@vbmade2000:~/Documents/projects/oss/util-linux/target/release$ ./lsns -p 0
        NS TYPE   NPROCS   PID USER       COMMAND
4026531834 time      106  2592 vbmade2000 /usr/bin/pipewire
4026531835 cgroup    106  2592 vbmade2000 /usr/bin/pipewire
4026531836 pid       105  2592 vbmade2000 /usr/bin/pipewire
4026531837 user      106  2592 vbmade2000 /usr/bin/pipewire
4026531838 uts       106  2592 vbmade2000 /usr/bin/pipewire
4026531839 ipc       106  2592 vbmade2000 /usr/bin/pipewire
4026531840 net       105  2592 vbmade2000 /usr/bin/pipewire
4026531841 mnt       103  2592 vbmade2000 /usr/bin/pipewire
4026532636 mnt         2  3719 vbmade2000 /snap/snapd-desktop-integration/391/usr/bin/user-session-helper /snap/snapd-desktop-integ
4026532637 mnt         0       root       
4026532638 mnt         1  9482 vbmade2000 /snap/firmware-updater/226/bin/firmware-notifier
4026532639 pid         1  4118 vbmade2000 /opt/zoho.com/ulaa/ulaa --type=zygote --crashpad-handler-pid=4110 --enable-crash-reporter
4026532640 net         1  4118 vbmade2000 /opt/zoho.com/ulaa/ulaa --type=zygote --crashpad-handler-pid=4110 --enable-crash-reporter
4026532820 mnt         0       root       
4026532879 mnt         0       root