couldn't set tty to ppp discipline invalid argument

参考：

http://pptpclient.sourceforge.net/howto-diagnosis.phtml#conventions

http://blog.chinaunix.net/uid-20609878-id-1915875.html

Couldn't set tty to PPP discipline: Invalid argument

Symptom: the following messages appear before connection is established:

Serial connection established.
Couldn't set tty to PPP discipline: Invalid argument
Hangup (SIGHUP)

Diagnosis: pppd has failed to change the pty over to run
it in PPP mode. This may be because you have no ppp_async
module built for your kernel. Most kernels are built with this
already, but if you have customised your kernel you may not have built
it.

Solution: rebuild your kernel with CONFIG_PPP_ASYNC set. While
you are at it, you should set CONFIG_PPP as well, and both should be
set to "m" so that they are built as modules. We've found they don't
work compiled statically.

重新配置内核（Device Drivers->Network device support->PPP support），把ppp相关的都选上。