Description:
------------
It seems like the behavior of starting a session has changed between < 7.1 and 7.1.

I'm not sure whether this is intentional (I couldn't find any mention of it in NEWS or UPGRADING or I just overlooked it).

When trying to start a session twice while the headers have already been sent I get:

Notice: A session had already been started - ignoring session_start() in /in/itgeJ on line 5

on the second session_start() call on 7.0 (and below), but on 7.1 I get:

Warning: session_start(): Cannot send session cookie - headers already sent by
Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /in/itgeJ:2) in /in/itgeJ on line 5

on the second session_start() call.

Test script: https://3v4l.org/NXiYR

Test script:
---------------
output
<?php

session_start();
session_start();

Expected result:
----------------
Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /in/NXiYR:2) in /in/NXiYR on line 4

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /in/NXiYR:2) in /in/NXiYR on line 4

Notice: A session had already been started - ignoring session_start() in /in/NXiYR on line 5

Actual result:
--------------
output

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /in/NXiYR:2) in /in/NXiYR on line 4

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /in/NXiYR:2) in /in/NXiYR on line 4

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /in/NXiYR:2) in /in/NXiYR on line 5

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /in/NXiYR:2) in /in/NXiYR on line 5