retain copy of required components;
more thorough purge_platforms;
tuned message;
clarified settings and defaults;
proper path before tar -C context switch;
replaced "isabelle components_checksum" shell script by "isabelle build_components" in Scala, with more functionality; more robust Component.Archive name: avoid rm_tree accidents;
clarified modules; --HG-- rename : src/Pure/System/components.scala => src/Pure/Admin/components.scala