WIP: add source contexts, remove debugger-only macros

WIP: add source contexts, remove debugger-only macros

 * a 'struct nobug_context' is used to pass information about sourcefile
   linenumber and function along.
 * many macros have an additional *_CTX variant which takes such a context
   (more to be implemented on demand)
 * general refactoring to more basic cases, less code duplication,
   prepare to move more things from macros to library functions.
 * remove *_DBG variants which where only active when running under a
   debugger, the flags mechanism provides more flexibility and
   superseeded these.
 * change the semantics of thread_id's, each thread gets on unique number
   now which never changes. Resetting the thread identifier increments a
   generation counter for the thread (currently unused)
 * don't depend on gcc's ## __VA_ARGS_ semantics. Found a way to pass
   ""__VA_ARGS__ instead is portable, the only remaining case is
   NOBUG_ASSERT_ which has a conditional fallback for other compilers
   (and then don't print the failed expression anymore)

The above things are not yet in the documentation.