Fix: JTAG scan irlens handling

[dragonmux]

Detailed description

Thanks to #1446 spurring us on, we have now addressed the way the manually specified IR lengths logic is handled as between the VLA usage, lack of length checking and other handling issues, the soundness of the IR lengths code was questionable.

This PR addresses this with a partial rewrite and finishes the refactoring work began in #1389 in jtag_scan() + cleans up the ST-Link v2 specific version of this routine. The result should be more consistent and robust with better user feedback.

This PR does slightly alter how the IR lengths are specified - dropping the need for a trailing 0 in the array, and making any 0's specified on the command line errors - but as this fixes a bug in how they could be specified and the user was not visibly aware of this behaviour/need, we think it's a safe change (changing only implementation details),

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Closing issues

[esden]

LGTM