client/mysql.cc:
  bug #5920 (embedded-server mysql doesn't handle --defaults-file)
  defaults files handled
include/my_sys.h:
  bug #5920 (embedded-server mysql doesn't handle --defaults-file)
  get_defaults_file interface added
libmysqld/libmysqld.c:
  just small fix, not related to the bug.
mysys/default.c:
  bug #5920 (embedded-server mysql doesn't handle --defaults-file)
  get_defaults_files implementation