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

src/applications/config/check/PhabricatorPHPConfigSetupCheck.php

@@ -224,5 +224,26 @@ final class PhabricatorPHPConfigSetupCheck extends PhabricatorSetupCheck {
         ->setSummary($summary)
         ->setMessage($message);
     }
+
+    $raw_post_data = (int)ini_get('always_populate_raw_post_data');
+    if ($raw_post_data != -1) {
+      $summary = pht(
+        'PHP setting "%s" should be set to "-1" to avoid deprecation '.
+        'warnings.',
+        'always_populate_raw_post_data');
+
+      $message = pht(
+        'The "%s" key is set to some value other than "-1" in your PHP '.
+        'configuration. This can cause PHP to raise deprecation warnings '.
+        'during process startup. Set this option to "-1" to prevent these '.
+        'warnings from appearing.',
+        'always_populate_raw_post_data');
+
+      $this->newIssue('php.always_populate_raw_post_data')
+        ->setName(pht('Disable PHP %s', 'always_populate_raw_post_data'))
+        ->setSummary($summary)
+        ->setMessage($message)
+        ->addPHPConfig('always_populate_raw_post_data');
+    }
   }
 }