Add a setup warning for "always_populate_raw_post_data"

Summary:
Fixes T9235. When the stars align, PHP 5.6 or newer emits a deprecation warning on startup about "always_populate_raw_post_data" which occurs too early for us to intercept and can break responses by adding garbage to the output.

These settings appear to be sufficient:

```
always_populate_raw_post_data = 1
display_errors = 1
display_startup_errors = 1
error_reporting = -1
```

Then make a request with an unusual content type:

```
$ curl -X POST -H "Content-Type: application/json" -d "{foo: bar}" http://phabricator.example.com/
```

This triggers the warning:

```
<br />
<b>Deprecated</b>: Automatically populating $HTTP_RAW_POST_DATA is deprecated and will be removed in a future version. To avoid this warning set 'always_populate_raw_post_data' to '-1' in php.ini and use the php://input stream instead. in <b>Unknown</b> on line <b>0</b><br />
<br />
...
```

To avoid this, just instruct administrators to set this value to "-1", which completely disables the feature and silences the warning.

Test Plan:
- Reproduced this issue by following the instructions above.
- Triggered the setup issue locally and read all the captivating prose:

{F1786911}

- Made the configuration change it directed me to, saw the setup issue resolve.

Reviewers: jcox

Reviewed By: jcox

Maniphest Tasks: T9235

Differential Revision: https://secure.phabricator.com/D16454