Add ps syscall and userspace ps command

[ankitpokhrel08]

I implemented a new ps feature in xv6 that exposes active process information from kernel space to userspace through a dedicated syscall.

This PR adds a full end-to-end syscall path
(userspace -> syscall stub -> syscall dispatcher -> kernel worker -> copyout -> userspace print)
and a simple ps command to validate behaviour from the shell.

What I changed

• Added a new syscall number for ps
• Added kernel syscall handler and dispatcher wiring
• Implemented kernel process walker that:
  • Iterates process table
  • Collects active processes SLEEPING, RUNNABLE, RUNNING
  • Copies process info safely back to userspace copyout
• Added shared process info struct for kernel-userspace contract
• Added userspace syscall declaration and stub generation
• Added userspace ps command
• Updated build config to include ps program in user binaries

Why this change
The goal was to implement a realistic syscall pattern and practice core OS concepts:

• argument passing from userspace to kernel
• safe memory transfer across the privilege boundary
• process table traversal with locking
• syscall dispatch integration
  Sample output

How I tested

• Built xv6 and booted in QEMU
• Ran ps at idle and verified base processes (init, sh, ps )
  
  
  
• Spawned background workloads (e.g. sleep) and confirmed they appear in ps
  
  
  
• Re-ran ps multiple times to confirm stable syscall behaviour and expected state transitions [After 10 sec]
  
  
  

Notes

• The command intentionally shows only active processes for clarity.
• ps appears as running while it is executing, which is expected.

[ankitpokhrel08]

Case Study on Operating System as a part of the Operating System course completed!

[ankitpokhrel08]

Also added a userspace sleep command sleep.c, sleep N now maps to roughly N seconds (tick-based via pause()), which makes ps demos much easier. You can now run sleep 1 & and immediately observe the process in SLEEPING state via ps.