adb: add `adb host-features`, report libusb status.

Add a 'host-features' command to get the features of the currently
running host adb server. Abuse it to report libusb status.

Bug: http://b/34983123
Test: adb host-features; adb kill-server; ADB_LIBUSB=1 adb start-server; adb host-features
Change-Id: I0e8d503a2dbdff9002ebb6ce8a298498a9421422
diff --git a/adb/usb.h b/adb/usb.h
index 879bacc..ba70de4 100644
--- a/adb/usb.h
+++ b/adb/usb.h
@@ -55,3 +55,5 @@
 
 // USB device detection.
 int is_adb_interface(int usb_class, int usb_subclass, int usb_protocol);
+
+bool should_use_libusb();