Took some time, but here is a patch that makes mmap(2) work on
Mac OS X.
This also applies to #11779.

Background:
on OS X, fsync(2) seems to behave as fdatasync(2).
To give people the possibility to do some kind of fync(2)
nonetheless, a new fcntl(2) has been introduced: F_FULLFSYNC.
If you use that, the ,sparse` file is synchronized with physical
backing store immediately and all is fine.
Vampire magic!