idle - make process 0 idle

#include <unistd.h>
[[deprecated]] int idle(void);

idle() is an internal system call used during bootstrap. It marks the process's pages as swappable, lowers its priority, and enters the main scheduling loop. idle() never returns.

Only process 0 may call idle(). Any user process, even a process with superuser permission, will receive EPERM.

idle() never returns for process 0, and always returns -1 for a user process.