Add PPTP support in mtpd.

If control socket does not exist, parse arguments from command line.
diff --git a/Android.mk b/Android.mk
index 78c5763..b0fd843 100644
--- a/Android.mk
+++ b/Android.mk
@@ -19,7 +19,7 @@
 LOCAL_PATH := $(call my-dir)
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES := mtpd.c l2tp.c
+LOCAL_SRC_FILES := mtpd.c l2tp.c pptp.c
 LOCAL_SHARED_LIBRARIES := libcutils
 LOCAL_CFLAGS := -DANDROID_CHANGES