Support for 3rd party USB Vendor IDs in adb.

Vendor IDs are read from ~/.android/adb_usb.ini. The format is very simple:
1 number per line. First number is ID count, followed by the ID themselves.
Lines starting with # are considered comments.

Other misc changes: moved VENDOR_ID_* to usb_vendors.c to prevent direct
access. Made transport_usb.c reuse the USB constant introduced in usb_osx
(moved them to adb.h)
diff --git a/adb.h b/adb.h
index 7762e00..3c4fe60 100644
--- a/adb.h
+++ b/adb.h
@@ -357,11 +357,10 @@
 #define ADB_PORT 5037
 #define ADB_LOCAL_TRANSPORT_PORT 5555
 
-// Google's USB Vendor ID
-#define VENDOR_ID_GOOGLE        0x18d1
+#define ADB_CLASS              0xff
+#define ADB_SUBCLASS           0x42
+#define ADB_PROTOCOL           0x1
 
-// HTC's USB Vendor ID
-#define VENDOR_ID_HTC           0x0bb4
 
 void local_init();
 int  local_connect(int  port);