service: accept the fact that the three xyz_good() functions return ints

service: accept the fact that the three xyz_good() functions return ints

Currently, all three of cgroup_good(), main_pid_good(),
control_pid_good() all return an "int" (two of them propagate errors).
It's a good thing to keep the three functions similar, so let's leave it
at that, but then let's clean up the invocation of the three functions
so that they always clearly acknowledge that the return value is not a
bool, but potentially negative.