Recognize the --no-defaults, --defaults-file and --defaults-extra-file
options.  Treat old --config-file argument as if --defaults-extra-file
had been specified instead.

Plus a few other defaults-related cleanups.


extra/my_print_defaults.c:
  
  Make help text more accurate regarding how --config-file, --defaults-file, and --defaults-extra-file should be used.  Flag --config-file as deprecated.
mysys/default.c:
  
  Always print a newline after listing the default files, even if
  --defaults-file= was set (in my_print_default_files()).
scripts/mysqld_multi.sh:
  
  Recognize --no-defaults, --defaults-file and --defaults-extra-file options.
  
  Treat old --config-file argument as if --defaults-extra-file had been specified
  instead.
  
  Improve find_groups() method, to honor --defaults-file, etc.
  
  A few random drive-by cleanups, while I'm here.