A big yet interim patch emerging from my attemp...

A big yet interim patch emerging from my attempts to centralize common
classloader-related code. As it turns out, not that much of the
patch is directly associated with that. Most of it is cleanup in the
neighborhoods I was visiting, but there are a few new library files
about which I'm open to feedback:

  scala/util/control/Exception - lots of exception handling code.
  scala/net/Utility - what would be the first file in scala.net.*,
    more code to follow if that sounds like a good package idea.
  scala/util/ScalaClassLoader - mostly convenience methods right
    now, more sophistication to come

Also, this adds a :jar command to the repl which adds a jar to your
classpath and replays the session.

Author: paulp

test/files/jvm/interpreter.check

@@ -171,7 +171,7 @@ scala> <console>:1: error: expected start of definition
 scala> 
 scala> 
 scala> 
-scala>      |      |      |      | res3: scala.xml.Elem = 
+scala>      |      |      |      | res2: scala.xml.Elem = 
 <a>
        <b d="dd" c="c"></b></a>
 
@@ -181,7 +181,7 @@ scala>      |      |      |      |
 scala> 
 scala> 
 scala> 
-scala>      |      |      | res4: java.lang.String = 
+scala>      |      |      | res3: java.lang.String = 
 
        hello
        there