systemctl: clean up start_unit_one() error handling

systemctl: clean up start_unit_one() error handling

Let's split exit code handling in two: "r" is only used for errno-style
errors, and "ret" is used for exit() codes. Then, let's use EXIT_SUCCESS
for checking whether the latter is already used.

This way it should always be clear what kind of error we are processing,
and when we propaate one into the other.

Moreover this allows us to drop "q" form all inner loops, avoiding
confusion when to use "q" and when "r" to store received errors.

Fixes: #9704