clarified modules;

clarified bootstrap;

back to dynamic conditional compilation (reverting 4764473c9b8d) via recursive ML name space;

tuned signature;

back to static conditional compilation -- simplified bootstrap;

clarified conditional compilation;

clarified files;

--HG--
rename : src/Pure/Concurrent/bash.ML => src/Pure/System/bash.ML
rename : src/Pure/Concurrent/bash.scala => src/Pure/System/bash.scala
rename : src/Pure/Concurrent/bash_windows.ML => src/Pure/System/bash_windows.ML