The intention of Packet_ref was to allow clients to place opaque
references into the packet descriptor itself, which could be observed on
packet completion. Currently no component in our sources uses this
feature and beyond that it is questionable if it should be used at all:
If the server tampers with the ref the client may easily be confused
into observing an incorrect or invalid context. It seems better to
remove the opaque context from the descriptor and leave the actual
implementation to the client and its needs.
Allow symlinks to be passed to the read and write file system utilities.
Disallow writes to symlinks with offsets in file system servers, this is
to ensure that writing the target of a symlink is an atomic operation.
Fixes#1604
