probe: block signals around USB transfers

[flit]

This patch blocks all signals from being sent to the current thread before performing any USB transfer through pyusb.
For the separate receive threads created by some CMSIS-DAP USB backends, signals are permanently blocked.

The main reason to do this is that a SIGINT can cause a KeyboardInterrupt exception to be raised in the middle of pyusb and/or pyocd USB protocol related code. On occasion, this will cause the protocol to become out of sequence, resulting in all future transfers failing wildly.

[flit]

Going to pause this one. More work is needed. A KeyboardInterrupt could still be raised from within (for instance) the CMSIS-DAP internal packet management code, which can break the sequencing of protocol transfer commands.

A real question is whether pyocd should even be doing this. Really the application should install its own SIGINT handler that completely avoids using KeyboardInterrupt (which pyocd does not do for its own gdb server, etc, right now!). Perhaps pyocd should just provide a utility to make this easy.