multiboot2: Follow C behavior for string parsing #761

Job	Run time
build (msrv) / rust	30s
build (nightly) / rust	39s
build (stable) / rust	32s
style (msrv) / rust	37s
build no_std (msrv) / rust	33s
build no_std (stable) [w/o builder] / rust	30s
build no_std (stable) / rust	31s
build no_std (stable) [Windows] / rust	1m 18s
style (stable) / rust	41s
tests with miri (nightly) / rust	1m 27s
build no_std (nightly) / rust	37s
style (nightly) / rust	36s
	8m 31s

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