* Don't use "usb_clear_halt" by default anymore
- It caused problems with specific devices in the past
- Now it cause problems with specific servers as well (64 bits, USB3)
* Add an "XTALK_OPTIONS" environment variable to pass options:
- Use it to implement a "use-clear-halt" boolean option that
restore original behavior.
- Also use it for "no-lock" option which replace the legacy
environment variable "XUSB_NOLOCK".
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