[RFC] job: Add optional {flags} parameter.

fixes #1574
Before I forget, can we add some verbiage here regarding the empty item. E.g.:

```
A buffered ('b') response always ends with an empty string item [''] to indicate the end of the output;
absence of a final empty string item indicates that the buffer overflowed. In contrast, unbuffered ('u')
data may emit any part of the output, so the lack of a final empty item indicates an unterminated line.
```