There are two places that QD keep trying to get data, ignore SIGINT, and
not send signal to QEs. If the program on segment has no input/output,
copy command hangs.

To fix it, this commit:

1, lets QD wait connections able to be read before PQgetResult(), and
cancels queries if gets interrupt signals while waiting
2, sets DF_CANCEL_ON_ERROR when dispatch in cdbcopy.c
3, completes copy error handling

-- prepare
create table test(t text);
copy test from program 'yes|head -n 655360';

-- could be canceled
copy test from program 'sleep 100 && yes test';
copy test from program 'sleep 100 && yes test<SEGID>' on segment;
copy test from program 'yes test';
copy test to '/dev/null';
copy test to program 'sleep 100 && yes test';
copy test to program 'sleep 100 && yes test<SEGID>' on segment;

-- should fail
copy test from program 'yes test<SEGID>' on segment;
copy test to program 'sleep 0.1 && cat > /dev/nulls';
copy test to program 'sleep 0.1<SEGID> && cat > /dev/nulls' on segment;