Fix/document incompatibility between -wait and stdin/stdout mode on server

[rossica]

the -wait switch allows the server to receive multiple files from senders. Support was partially implemented in #32 to allow this to mean that multiple, sequential, stdin/stdout connections could be made to the server before it would exit.
The issue is that the thread which reads from stdin on the server assumes the connection structure stays around forever. A solution needs to be devised to make that thread agnostic of which connection is available.
Or the documentation (what documentation?) needs to be updated to reflect this limitation. Make this part of #16.