Clean up hostfs TODOs.
Clean up hostfs TODOs.

This CL does a handful of things:
- Support O_DSYNC, O_SYNC
- Support O_APPEND and document an unavoidable race condition
- Ignore O_DIRECT; we probably don't want to allow applications to set O_DIRECT
  on the host fd itself.
- Leave a TODO for supporting O_NONBLOCK, which is a simple fix once RWF_NOWAIT
  is supported.
- Get rid of caching TODO; force_page_cache is not configurable for host fs in
  vfs1 or vfs2 after whitelist fs was removed.
- For the remaining TODOs, link to more specific bugs.

Fixes #1672.

PiperOrigin-RevId: 317985269

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor