Implement All xv6 Syscalls

Created by: tjhu

Implement all of the of xv6 syscalls and their proxies in rv6.

1. SYS_fork(merge with exec as sys_spawn_domain)
   • syscall
   • proxy
2. SYS_exit(need discussion)
   • syscall
   • proxy
3. SYS_wait(implemented as Thread::join)
   • syscall
   • proxy
4. SYS_pipe
   • syscall
   • proxy
5. SYS_read
   • syscall
   • proxy
6. SYS_kill
   • syscall
   • proxy
7. SYS_exec
   • syscall
   • proxy
8. SYS_fstat
   • syscall
   • proxy
9. SYS_chdir
   • syscall
   • proxy
10. SYS_dup
    • syscall
    • proxy
11. SYS_getpid
    • syscall
    • proxy
12. SYS_sbrk(no need; we use memory allocator)
    • syscall
    • proxy
13. SYS_sleep(for ns instead for ticks)
    • syscall
    • proxy
14. SYS_uptime
    • syscall
    • proxy
15. SYS_open
    • syscall
    • proxy
16. SYS_write
    • syscall
    • proxy
17. SYS_mknod
    • syscall
    • proxy
18. SYS_unlink
    • syscall
    • proxy
19. SYS_link
    • syscall
    • proxy
20. SYS_mkdir
    • syscall
    • proxy
21. SYS_close
    • syscall
    • proxy