Consolidate & enhance "environment config" handling.

The new code is more careful about how it obtains config info, and issues
warnings when sources of info conflict. It does not abort in such circumstances
as Getdown's prime directive is to do the best it can to get the app updated
and launched. But the warnings will hopefully allow accidental conflicts to be
detected and avoided before being deployed to customers.

We also now allow bootstrap.properties to provide an appid; that matches what
was possible via system properties and command line arguments. We also avoid
"communicating" environment configuration through the Java system properties
mechanism. We now read from sysprops but do not write back to them.

Lastly, this includes some name tidying (like appdir -> appDir and
getAppdir -> getAppDir).

core/src/test/java/com/threerings/getdown/classpath/ClassPathsTest.java

@@ -28,7 +28,7 @@ public class ClassPathsTest
         when(_firstJar.getFinalTarget()).thenReturn(_firstJarFile);
         when(_secondJar.getFinalTarget()).thenReturn(_secondJarFile);
         when(_application.getActiveCodeResources()).thenReturn(Arrays.asList(_firstJar, _secondJar));
-        when(_application.getAppdir()).thenReturn(_appdir.getRoot());
+        when(_application.getAppDir()).thenReturn(_appdir.getRoot());
     }
 
     @Test public void shouldBuildDefaultClassPath () throws IOException