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

updated to jdk-8u152, which is for x86_64 only;

renamed jvmpath to platform_path;

clarified ISABELLE_JDK_HOME vs. JAVA_HOME;

absorb final CLASSPATH as well, such that tools might provide that by elementary means, without the "classpath" shell function (e.g. kodkodi/nitpick);

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;

provide ISABELLE_JAVA_SYSTEM_OPTIONS via settings;

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

more robust invocation via ISABELLE_JDK_HOME and SCALA_HOME;

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

ISABELLE_JDK_HOME settings variable points to JDK with javac and jar (not just JRE);
update for prospective jdk1.7.x component;

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

more explicit check of Java executable -- relevant for Linux x86/x86_64 mismatch and absence on Mac OS Lion;

provide Isabelle/Scala environment as Java extension, instead of user classpath
(which is subject to adhoc changes);

java/scala: default to UTF-8;

isabelle java: prefer -server here;

recovered isabelle java, which was broken in ebfa4bb0d50f;

refer to THIS_JAVA dynamically, and treat ISABELLE_JAVA as static default -- relevant for nested JVM invocation within an existing Isabelle enviroment;

removed Ids;

removed unused usage;

refined scala/java wrappers via isatool;