* lisp/net/tramp.el (tramp-get-buffer): Set connection property
"process-buffer" in order to mark connection as active.
* lisp/net/tramp-cache.el (tramp-get-hash-table)
(tramp-set-connection-property)
(tramp-dump-connection-properties): Do not use "active" property.
(tramp-list-connections): Use "process-buffer" property.
* lisp/net/tramp-cmds.el (tramp-cleanup-connection): Flush process
properties prior deletion of process.