Add optional stat arguments to co_sum, co_min, & co_max

[ktras]

@rouson In caf_co_max_test.f90 you did not assign the variable status a value before passing it to the collective, whereas in the other tests, you did so. Did you mean for caf_co_max_test.f90 to differ like that? If that was your intent, then this looks good to me.

[rouson]

@ktras great catch. I meant to initialize it to ensure that it gets redefined to 0 upon exiting each invoked C function.

[rouson]

I just pushed a fix and will merge after tests pass.