mqueue: fix mq_size() not to return 0

- return 0 for a config function is considered 'exit' by interpreter
- if size of the queue is 0, the function return -1
- for other error cases, the return code was decremented by 1

(cherry picked from commit bde57b53386e792651b5ac86ea32d16ab721a07c)
mqueue: elaborated the docs for mq_size() to reflect return codes

(cherry picked from commit cf9303b8063e77426e0506bed09577fe7a584fb1)