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This change syncs external/ppp with upstream. Specifically it's based on
upstream's 6e8eaa7a78b31cdab2edf140a9c8afdb02ffaca5.

In order to do this without blowing away the Android-specific changes, I
ran a script to try and find a matching revision in upstream's git tree
to the initial commit in external/ppp.

Based on the contents of *.c and *.h files in pppd/, upstream's
f8583676904957554c5a8bd14ede415688835f6c matches with the local revision
82c907af.

I diffed upstream's f8583676 with upstream's HEAD and removed chunks
from the result that applied to files that aren't present in Android. In
some places, the diff didn't apply cleanly because of Android-specific
changes:

  1) Local change 8ad0dd2a added two "return UPAP_AUTHNAK;" statements
     in "#if 1" blocks in auth.c. They have been reflected into
     session.c/session_start by always returning SESSION_FAILED.
     (Additionally, new code to query the local password db has been
     disabled...