constrain signal delivery to Scheme to the main thread #230

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	29m 0s
ci (a6osx, macos-12)	13m 59s
ci (ta6osx, macos-12)	21m 18s
ci (arm64osx, macos-14)	8m 12s
ci (tarm64osx, macos-14)	10m 42s
ci (i3le, ubuntu-22.04)	11m 24s
ci (ti3le, ubuntu-22.04)	14m 46s
ci (a6le, ubuntu-22.04)	9m 0s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	11m 51s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 44s
ci (i3nt, windows-2022, vs)	11m 28s
ci (ti3nt, windows-2022, vs)	15m 3s
ci (a6nt, windows-2022, vs)	11m 33s
ci (ta6nt, windows-2022, vs)	14m 52s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 45s
	3h 25m 37s

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