Move adb to C++.

I keep trying to clean things up and needing std::strings. Might as
well just do this now.

usb_linux_client.c is going to stay as C because GCC isn't smart
enough to deal with the designated initializers it uses (though for
some reason it is in C mode).

The Darwin files are staying as C because I don't have a way to test
that they build.

The Windows files are staying as C because while I can actually build
for them, it's slow and painful.

Change-Id: I75367d29205a9049d34460032b3bb36384f43941
diff --git a/adb/transport.h b/adb/transport.h
index 352bbe4..36a0e40 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -37,7 +37,7 @@
  * If no suitable transport is found, error is set.
  */
 atransport* acquire_one_transport(int state, transport_type ttype,
-                                  const char* serial, char** error_out);
+                                  const char* serial, const char** error_out);
 void add_transport_disconnect(atransport* t, adisconnect* dis);
 void remove_transport_disconnect(atransport* t, adisconnect* dis);
 void kick_transport(atransport* t);