Remove obsolete coding for early macOS.

Commits 04cad8f7 and 0c088568 supported old macOS systems that didn't
define O_CLOEXEC or O_DSYNC yet, but those arrived in macOS releases
10.7 and 10.6 (respectively), which themselves reached EOL around a
decade ago.  We've already made use of other POSIX features that early
macOS vintages can't compile (for example commits 623cc673, d2e15083).

A later commit will use O_CLOEXEC on POSIX systems so it would be
strange to pretend here that it's optional, and we might as well give
O_DSYNC the same treatment since the reference is also guarded by a test
for a macOS-specific macro, and we know that current Macs have it.

Discussion: https://postgr.es/m/CA%2BhUKGKb6FsAdQWcRL35KJsftv%2B9zXqQbzwkfRf1i0J2e57%2BhQ%40mail.gmail.com

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