assignment context to retrieve both the return value (if any) and the
C errno variable as an error (use _ to skip the result value if the
function returns void). For example:

	n, err = C.sqrt(-1)
	_, err := C.voidFunc()
	var n, err = C.sqrt(1)

Note that the C errno value may be non-zero, and thus the err result may be
non-nil, even if the function call is successful. Unlike normal Go conventions,