Add libteken (from FreeBSD) to third_party.

third_party/libteken/README.chromium

Index: third_party/libteken/README.chromium
diff --git a/third_party/libteken/README.chromium b/third_party/libteken/README.chromium
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+Name: libteken
+URL: svn://svn.FreeBSD.org/base/head/sys/teken/
+Version: r276292
+License: BSD
+License File: COPYRIGHT
+Security Critical: no
+
+Description:
+This is libteken from FreeBSD, a VT100-ish terminal emulator library.
+
+It was obtained as follows (from this directory):
+
+  svn export svn://svn.FreeBSD.org/base/head/sys/teken/@276292
+  svn export svn://svn.FreeBSD.org/base/head/COPYRIGHT@276292
+
+Local modifications:
+* teken/teken_state.h: manually generated (from the teken subdirectory) using:
+  |awk -f gensequences sequences > teken_state.h|.
+* teken/teken.h: |#include <stdint.h>|.
+* teken/teken.h: |extern "C"| guards added for C++.
+* teken/gensequences: made executable, to placate checkperms.py.