make it return an error (except if certain errno), test this error in
callers. Do a single my_sync_dir() in my_rename() if possible.


include/my_global.h:
  better have a symbol name talking about the feature, use it in the
  code of the feature, and define the symbol once depending on the
  platform, rather than have the platform "tested" in the code
  of the feature several times.
include/my_sys.h:
  my_sync_dir() now can return error
mysys/my_create.c:
  my_sync_dir() can now return an error
mysys/my_delete.c:
  my_sync_dir() can now return an error
mysys/my_rename.c:
  my_sync_dir() can now return an error.
  Do a single sync if "from" and "to" are the same directory.
  #ifdef here to not even compile dirname_part() if useless.
mysys/my_sync.c:
  more comments.
  A compilation error if no way to make my_sync() work (I guess
  we don't want to ship a binary which cannot do any sync at all;
  users of strange OSes compile from source and can remove
  the #error).
  my_sync_dir() now returns an error (except for certain errno values
  considered ok; EIO "input/output error" is not ok).
sql/unireg.cc:
  my_sync_dir() now returns an error which must be tested