Avoid getattrlist TOCTOU issue for atomic file writing

[jmschonfeld]

Use FSOPT_UNIQUE in getattrlist when writing files atomically

Motivation:

getattrlist can provide stale information due to TOCTOU behavior exhibited by the kernel. To avoid this issue, we can use FSOPT_UNIQUE to have the kernel fail for us if there are multiple hard link locations rather than checking the returned data ourselves

Modifications:

Adds FSOPT_UNIQUE to the getattrlist calls in data atomic writing and remove the nlink check of the result

Result:

We no longer read potentially stale information from the getattrlist result

Testing:

This is a small window for a race condition in the kernel that is not deterministically testable. It is also in FOUNDATION_FRAMEWORK code

[jmschonfeld]

@swift-ci please test