DiffieHellman: gracefully handle platforms that fail the sha256 self test, currently only known to be PowerPC. Added support for PHP's built-in hash functions (they were added in 5.1.2) and use them in lieu of the built-in sha256 implementation when available.

# Used for testing mod_rewrite.
<IfModule mod_rewrite.c>
RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule (.*) install.php?do=modrewrite_test&str=$1
</IfModule>