Add --type option in lsns utility.

[vbmade2000]

Closes #606

myuser@mysystem:~/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)
  -h, --help         Print help
  -V, --version      Print version

myuser@mysystem:~/util-linux/target/release$ ./lsns --type mnt
        NS TYPE NPROCS   PID USER       COMMAND
4026531841 mnt     103  3248 myuser /usr/bin/pipewire
4026532636 mnt       0       root       
4026532763 mnt       1 37039 myuser /snap/firmware-updater/226/bin/firmware-notifier
4026532820 mnt       0       root       
4026532879 mnt       0       root       
4026532887 mnt       2  5317 myuser /snap/snapd-desktop-integration/391/usr/bin/user-session-helper /snap/snapd-desktop-integration/391/usr/bin/snapd-desktop-integration