Add support for macOS fcntl(F_NOCACHE, 1)

[laurynas-biveinis]

A macOS counterpart of O_DIRECT. This should be helpful for e.g. TokuDB and RocksDB testsuites.

[stewartsmith]

Implemented in 1e5ae05

[laurynas-biveinis]

It's something separate from F_FULLSYNC patch and thus it's not implemented there - please reopen?

[stewartsmith]

Helps if I read the issue I close. Whoops :-)

[stewartsmith]

So, I finally have a Mac to actually go and poke at these kind of things on (okay, I could have gone and spawned an EC2 instance... but laziness prevailed!).

One of the challenges with masking out O_DIRECT is that there are semantic changes that you lose without a whole bunch of extra checks.

Is there anything that F_NOCACHE would do? I can't see anything in the man page...