Fix `FdWriter::ReadMode()` support detection when `FdWriter` gets an already open fd and `Options::assumed_pos()` is used.
Fix `FdWriter::ReadMode()` support detection when `FdWriter` gets an already
open fd and `Options::assumed_pos()` is used.

This used to be a case when `fcntl()` could be skipped because it did not matter
for checking `O_APPEND`, but now it is used also to check for `O_RDWR`, so the
optimization is no longer valid.

PiperOrigin-RevId: 479515804

google/riegeli,google/riegeli,google/riegeli,google/riegeli