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