Implement pwritev2 and associated unit tests.
Clean up preadv2 unit tests.
Tag RWF_ flags in both preadv2 and pwritev2 with associated bug tickets.
PiperOrigin-RevId: 226222119
Change-Id: Ieb22672418812894ba114bbc88e67f1dd50de620
This allows ValueOrDie() to be called on PosixErrorOr rvalues (e.g.
temporaries) holding move-only types without extraneous std::move()s.
PiperOrigin-RevId: 225098036
Change-Id: I662862e4f3562141f941845fc6e197edb27ce29b