Remove magic around string urls.

String urls no longer magically get the controller, plugin, or prefix
applied to them.  You only get the basedir applied.  This simplifies the
code and removes possibly un-expected magic from usage.

Start to carve out where named routes will fit.

lib/Cake/Routing/Router.php

@@ -636,6 +636,13 @@ public static function url($url = null, $options = false) {
 		}
 
 		$output = $frag = null;
+		$hasColonSlash = false;
+		$hasLeadingSlash = false;
+
+		if (is_string($url)) {
+			$hasColonSlash = strpos($url, '://') !== false;
+			$hasLeadingSlash = isset($url[0]) ? $url[0] === '/' : false;
+		}
 
 		if (empty($url)) {
 			$output = isset($here) ? $here : '/';
@@ -693,32 +700,21 @@ public static function url($url = null, $options = false) {
 				'plugin' => $params['plugin']
 			);
 			$output = self::$_routes->match($url, $params);
-		// } elseif (is_string($url) && $url[0] !== '/') {
+		} elseif (is_string($url) && !$hasLeadingSlash && !$hasColonSlash) {
 			// named route.
 
 		} else {
 			// String urls.
 			if (
-				(strpos($url, '://') !== false ||
+				($hasColonSlash ||
 				(strpos($url, 'javascript:') === 0) ||
 				(strpos($url, 'mailto:') === 0)) ||
 				(!strncmp($url, '#', 1))
 			) {
 				return $url;
 			}
-			if (substr($url, 0, 1) === '/') {
+			if ($hasLeadingSlash) {
 				$output = substr($url, 1);
-			} else {
-				foreach (static::$_prefixes as $prefix) {
-					if (isset($params[$prefix])) {
-						$output .= $prefix . '/';
-						break;
-					}
-				}
-				if (!empty($params['plugin']) && $params['plugin'] !== $params['controller']) {
-					$output .= Inflector::underscore($params['plugin']) . '/';
-				}
-				$output .= Inflector::underscore($params['controller']) . '/' . $url;
 			}
 		}
 		$protocol = preg_match('#^[a-z][a-z0-9+-.]*\://#i', $output);