Subcmd `send` cannot send >1022 chars on macOS

[clarkwang]

In #45:

On macOS, write(pty_fd, buf, len) in non-blocking mode would always return 1022 if len > 1022. Seems like the internal buffer is limited to 1024 bytes. Need to handle this with retry, also needs a timeout here or the server side would be blocking.

[clarkwang]

• The retry should happen in client side to keep server side logic as simple as possible.