more robust java.ext.dirs: avoid picking up accidental jars from system directories;

renamed jvmpath to platform_path;

maintain classpath in more elementary manner: turn ISABELLE_CLASSPATH into -classpath option, so that all jars are covered by sun.misc.Launcher.AppClassLoader (e.g. relevant for loading add-on resources);
ignore $ISABELLE_JAVA_EXT -- do not change java.ext.dirs;

more formal isabelle_admin_build;
tuned;

simplified JVM startup, based on stronger assumptions about the Java version provided as Isabelle component;

enforce our JAVA_HOME to avoid potential conflicts with other Java installations by the user;

direct scala toplevel tools to ISABELLE_JDK_HOME;

more robust command invocation via ISABELLE_JDK_HOME or SCALA_HOME (NB: bash exec requires genuine executable, not function);

offline build of java_ext_dirs.jar, to avoid runtime dependency on javac/jar executables;

--HG--
rename : src/Pure/System/Java_Ext_Dirs.java => src/Tools/JVM/Java_Ext_Dirs.java
rename : lib/scripts/java_ext_dirs => src/Tools/JVM/java_ext_dirs

added "isabelle scalac" convenience;