Read feature flag (send back to MOS)

[stevesims]

it could be useful to have a command to read a feature flag and send the value back to MOS

it may be worth considering just baking this functionality into the "set feature flag" command. the command could echo back the value that was set. the VDP could, potentially, adjust the flag value rather than just simply storing the value sent in the command, so it may be sensible/useful to return the new value.

the message could, potentially, return both previous and new values. but what should be sent if there was no previous value?

perhaps sending a flag identifier with the top bit set, and ignoring the value, could perform a pure/simple "read"