Allow Shared CLIENTs to use Shadow Files (instead of Server)

[Fish-Git]

Currently, only a Shared Server can specify Shadow files, not clients.

When a Shared Server attempts to open a read-only dasd without a shadow file specified, an error occurs.

Similarly, when a Shared Client attempts to specify a shadow file on any of its remote dasds (where the actual dasd exists on a Shared Server), an error also occurs.

It would be nice if one could start a Shared Client instance where the remote dasd was read-only on the Shared Server, but had shadow files specified for it, such that all of the client's writes then went to its own shadow file, and the remote dasd on the Server was only ever accessed in read-only mode (such as when the client is searching for a track that doesn't exist in any of its shadows).

In this way, the "base" read-only dasd could exist only on the server, but you could have multiple clients connected to it, each one with their own unique set of shadow files.

Note that this can be accomplished today without using the Shared Device Server. Each system can specify the same read-only dasd in its configuration file, but each specifying their own unique set of shadow files. Unfortunately however, the same thing cannot currently be done when using a Shared Device Server. It would be nice if you could though.