Extend `sessionctl` with user and seat commands #2

currently sessionctl only has some session operations implemented, it'd be nice to extend it with user and seat commands

some ideas for user related commands:

• list users (#4)
• lingering (#4)
• user status
• terminate user (#9)
• kill user (#9)

ideas for seat related commands:

• list seat (#10)
• seat status (#10)
• terminate seat (#10)

some session cmd improvements:

• lock all sessions (#11)
• unlock all sessions (#11)
• session status