Cache the results of wfIsWindows()

Cache the results of wfIsWindows()
Each php_uname() call produces a uname syscall.

The cached one is three times faster (3.197545885) which is liklely to be the difference between a php var lookup and a syscall on my system.

== Test script ==
<?php

function wfIsWindows() {
	if ( substr( php_uname(), 0, 7 ) == 'Windows' ) {
		return true;
	} else {
		return false;
	}
}

function wfIsWindowsCached() {
	static $isWindows = null;
	if ( $isWindows === null ) {
		$isWindows = substr( php_uname(), 0, 7 ) == 'Windows';
	}
	return $isWindows;
}

$win = $nonwin = 0;

$time = microtime( true );
for ( $i = 1; $i < 5e8; $i++ ) {
	if ( wfIsWindowsCached() ) {
		$win++;
	} else {
		$nonwin++;
	}
}

$time = microtime( true ) - $time;
echo "Time elapsed: $time\n";