constrain signal delivery to Scheme to the main thread (#813) #231

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	28m 36s
ci (a6osx, macos-12)	14m 22s
ci (ta6osx, macos-12)	22m 25s
ci (arm64osx, macos-14)	8m 15s
ci (tarm64osx, macos-14)	10m 11s
ci (i3le, ubuntu-22.04)	12m 21s
ci (ti3le, ubuntu-22.04)	14m 43s
ci (a6le, ubuntu-22.04)	9m 3s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	11m 3s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 18s
ci (i3nt, windows-2022, vs)	11m 26s
ci (ti3nt, windows-2022, vs)	14m 22s
ci (a6nt, windows-2022, vs)	11m 14s
ci (ta6nt, windows-2022, vs)	14m 43s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 31s
	3h 24m 33s

Provide feedback