MINOR: tools: make fddebug() automatically emit the location

MINOR: tools: make fddebug() automatically emit the location

fddebug() is sometimes quite helpful, but annoying to use when following
a call path because it's a pain to always repeat the function name and
call place. Let's have it automatically prepend the function name, the
file name and the line number, and make its arguments optional, replacing
them by a simple LF when all absent. This way, simply placing:

    fddebug();

is sufficient to emit a location follocing "[%s@%s:%d]\n". This function
must not be used in production (and even call places with it shouldn't be
committed) and it should only be used by developers, so the simplest the
better.