NIOCore: extract and modify POSIX flag constants (#2150)
NIOCore: extract and modify POSIX flag constants (#2150)

Windows does not have the same set of POSIX flag constants.  In
particular, there is no equivalent to group permissions as the ACL model
on Windows is far richer and requires the full specification of the
DACLs.  Additionally, isolate `O_CLOEXEC` to aid Windows which does not
have this flag.

Co-authored-by: Cory Benfield <e2b4f0650e369574e9b0029c3b3893634b6d4097@apple.com>
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio