22061 - stdin discovery plugin + use in rpcutil

This makes a plugin for discovery by stdin, which can
autodetect a list of hosts (in the same manor as flatfile)
or JSON (as per mco rpc)

Change over mco rpc so that it's automatic discovery of piped
json (for discovery data) uses this stdin plugin (and can now
also accept a flat \n seperated identity list equally well) + fix
bugs when using mco rpc with no tty (e.g. ssh somehost mco rpc ....)