cstrans-df-run: introduce the --shell-form option #278

Job	Run time
Ubuntu 20.04 (clang++)	1m 49s
Ubuntu 22.04 (clang++ with sanitizers)	3m 37s
Ubuntu 20.04 (g++)	1m 26s
Ubuntu 22.04 (g++ with sanitizers)	3m 52s
	10m 44s

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