RawOs marker traits

Currently, the {As,From}Raw{Fd,Handle} traits don't provide any information about how the file descriptors might safely be used (and don't even guarantee that the file descriptor is valid). Therefore, I would like to propose traits like:

/// Can safely write
unsafe trait RawOsWrite {}
/// Can safely read
unsafe trait RawOsRead {}
/// Can safely seek
unsafe trait RawOsSeek {}
/// Can safely access metadata. May not be necessary (always safe)?
unsafe trait RawOsMeta {}

These traits, when implemented on a type implementing one of the AsRaw/IntoRaw traits, would guarantee that the specified operation can be performed on the file descriptor/handle without causing memory unsafety. This doesn't mean that the operation will succeed, just that it won't be unsafe.

Unresolved: For AsRaw*, it's unclear when this guarantee expires. Personally, I'd like to say that the guarantee holds until the file descriptor owner is dropped but I don't know if that's reasonable.

Start by reviewing the existing AsRaw, FromRaw, AsRawFd, and AsRawHandle traits and the proposed RawOsWrite, RawOsRead, RawOsSeek, and RawOsMeta markers. Done means resolving the validity and AsRaw lifetime guarantees, deciding which operations each marker covers, and documenting the resulting trait design in an RFC.