darwin: fix fsync and fdatasync
darwin: fix fsync and fdatasync

Apple's fsync and fdatasync explicitly do NOT flush the drive write
cache to the drive platters. This is in contrast to Linux's fsync and
fdatasync which do, according to recent man pages. F_FULLFSYNC is
Apple's equivalent for flushing buffered data to permanent storage.

PR-URL: https://github.com/libuv/libuv/pull/1124
Refs: https://github.com/nodejs/node/issues/9439
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>

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