(execute): Clients can't reliably detect where a redirection ends

(execute): Clients can't reliably detect where a redirection ends
because the server doesn't send C_PROMPT.  The client simply rejects
such redirections since servcmd.c rev. 1.42.  Before it didn't, but
incorrectly extended the redirection until the end of the script, and
screwed up when there was more than one redirection in the execute.
Make the server reject such redirections as well, because that's
probably more useful and certainly less painful than documenting this
mess.