Add support for piped input with builtin commands

[YilinGao]

When some piped commands are builtin commands (i.e., cd, set, export, and exit), the pipe still doesn't work.

[YilinGao]

While drew says "make a pipe into which no output is placed before it is closed", I have no idea about how to do this....

And the bug seems to be intermittent, and I'm not sure if it still exists....