Add sys_dup && support input/output redirection in user_shell

user/src/lib.rs

@@ -69,6 +69,7 @@ bitflags! {
     }
 }
 
+pub fn dup(fd: usize) -> isize { sys_dup(fd) }
 pub fn open(path: &str, flags: OpenFlags) -> isize { sys_open(path, flags.bits) }
 pub fn close(fd: usize) -> isize { sys_close(fd) }
 pub fn pipe(pipe_fd: &mut [usize]) -> isize { sys_pipe(pipe_fd) }