[Wilds] Use "random_bytes()" if it is available (after PHP7)

Summary:
Ref T13209. In PHP7 and newer, the function `random_bytes()` gives us simple access to cryptographic randomness across platforms. Prefer it if it's available.

Notably, the other sources often aren't available on Windows, and particularly aren't available on a clean/default install with modern software versions. So the major motivation is to make things work better out-of-the-box on Windows.

Test Plan: On Windows, saw Filesystem unit tests which call `readRandomBytes()` now pass.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13209

Differential Revision: https://secure.phabricator.com/D19726

src/filesystem/Filesystem.php

@@ -421,6 +421,12 @@ final class Filesystem extends Phobject {
       throw new Exception(pht('You must generate at least 1 byte of entropy.'));
     }
 
+    // Under PHP7, the "random_bytes()" function provides a simpler and more
+    // portable way to accomplish what the rest of this function accomplishes.
+    if (function_exists('random_bytes')) {
+      return random_bytes($number_of_bytes);
+    }
+
     // Try to use `openssl_random_pseudo_bytes()` if it's available. This source
     // is the most widely available source, and works on Windows/Linux/OSX/etc.
 
@@ -481,9 +487,8 @@ final class Filesystem extends Phobject {
         pht(
           '%s requires the PHP OpenSSL extension to be installed and enabled '.
           'to access an entropy source. On Windows, this extension is usually '.
-          'installed but not enabled by default. Enable it in your "s".',
+          'installed but not enabled by default. Enable it in your "php.ini".',
-          __METHOD__.'()',
+          __METHOD__.'()'));
-          'php.ini'));
     }
 
     throw new Exception(