[AFS3-std] Request for a capability bit for RxOSD

[Felix Frank]

For OpenAFS+OSD, it would be helpful for the clients to identify OSD 
capable fileservers early. Codepaths can then be separated more easily 
for OSD access vs. normal fileserver access.

Similarly, fileservers can store a hint about clients that are OSD 
capable, and send OSD-related data only to those. Other clients will (as 
is the case already) be forwarded actual file data instead of being 
supplied with OSD metadata.