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/commandline.cpp b/adb/commandline.cpp
index 3b2df2e..5a2206f 100644
--- a/adb/commandline.cpp
+++ b/adb/commandline.cpp
@@ -1929,8 +1929,7 @@
     else if (!strcmp(argv[0], "--version") || !strcmp(argv[0], "version")) {
         fprintf(stdout, "%s", adb_version().c_str());
         return 0;
-    }
-    else if (!strcmp(argv[0], "features")) {
+    } else if (!strcmp(argv[0], "features")) {
         // Only list the features common to both the adb client and the device.
         FeatureSet features;
         std::string error;
@@ -1945,6 +1944,8 @@
             }
         }
         return 0;
+    } else if (!strcmp(argv[0], "host-features")) {
+        return adb_query_command("host:host-features");
     } else if (!strcmp(argv[0], "reconnect")) {
         if (argc == 1) {
             return adb_query_command("host:reconnect");