feat: add system reboot and process teardown support

The patch introduces support for system reboot functionality and process teardown,
allowing for a clean shutdown and unmounting of the root filesystem. This is
necessary for ensuring a proper system shutdown process, especially when dealing
with resource cleanup and ensuring that all processes have exited before system
shutdown.

Changes:
- Added `lwp_teardown()` function to handle process cleanup and system teardown.
- Introduced `lwp_pid_wait_for_empty()` to wait for process ID table emptiness
  before proceeding with shutdown.
- Updated `dfs_mnt_unref()` to trigger callbacks when unmounting a filesystem.
- Added new reboot types (`RB_AUTOBOOT`, `RB_POWER_OFF`) and implemented their
  corresponding actions, including cleanup of processes and unmounting root
  filesystem.
- Extended `sys_reboot()` to handle reboot and power off types with appropriate
  callbacks for process and filesystem teardown.
- Introduced callback mechanism for root filesystem unmount notifications.

Signed-off-by: Shell <[email protected]>

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