Allow setting of system properties from context.xml in addition to setting from command-line.

Accomplished by centralizing calls to System.getProperty(String) and related into new file SystemProperties.java.
There, we first check for existence of system property; if not, we look for property in context.xml
This is done for "application" properties (like hudson.DNSMultiCast.disabled) but not for java properties (like user.name)

core/src/main/java/hudson/Main.java

@@ -218,10 +218,10 @@ public class Main {
     /**
      * Set to true if we are running inside "mvn hpi:run" or "mvn hudson-dev:run"
      */
-    public static boolean isDevelopmentMode = Boolean.getBoolean(Main.class.getName()+".development");
+    public static boolean isDevelopmentMode = SystemProperties.getBoolean(Main.class.getName()+".development");
 
     /**
      * Time out for socket connection to Hudson.
      */
-    public static final int TIMEOUT = Integer.getInteger(Main.class.getName()+".timeout",15000);
+    public static final int TIMEOUT = SystemProperties.getInteger(Main.class.getName()+".timeout",15000);
 }