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
8 files changed