The idevicedebugserverproxy command takes a port number without a
flag, like so:

idevicedebugserverproxy 3222

If the -u <device_id> flag is added afterwards, it is ignored and
the command use an arbitrary device. Instead, always inject the -u
flag before any other idevice command arguments.

While here, also kill any leftover idevicedebugserverproxy instance
previous (failed) runs might have left running.

Change-Id: I0bf06ed1a20ef225abeca183f9ba8f396662d435
Reviewed-on: https://go-review.googlesource.com/112216
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>