ci-demand: a config file with a UTF-8 BOM is refused #122

A ci-demand config file saved as UTF-8 with a byte-order mark (some Windows editors do this) is refused with exit 2.

Fix: read the config as utf-8-sig (or strip the BOM) and add a test. A failing test was written during the review.

From the quick-check review of #113: #113 (comment)

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