Jilles says, in <slrneqpvd9.1scp.jilles@snail.stack.nl>:

The following seems specific to 4.4BSD derivatives (FreeBSD, NetBSD,                                                    
OpenBSD, DragonflyBSD, MacOS X, etc):                                                                                   
                                                                                                                        
The definition of when the discard (also: flush) (def. ^O) and lnext                                                    
(def. ^V) control characters are recognized is somewhat strange. They are                                               
recognized when the IEXTEN flag is set. So many usual sets of terminal modes                                            
will leave them enabled.                                                                                                
                                                                                                                        
When using xterm, it is possible to correct this on startup, for example                                                
using                                                                                                                   
                                                                                                                        
  xterm -xrm '*xterm*ttyModes: flush ^- lnext ^- status ^t' -fa VeraMono                                                
                                                                                                                        
This also enables the status character which allows ctrl+t when it is                                                   
trying to connect to give a status report (this does not affect actual                                                  
gameplay).                                                                                                              
                                                                                                                        
With any terminal, you can use commands like                                                                            
                                                                                                                        
  (stty discard undef lnext undef status ^t; telnet crawl.akrasiac.org)                                                 
                                                                                                                        
Doing it in a single line avoids problems with shells that revert                                                       
terminal mode changes.                                                                                                  
                                                                                                                        
--                                                                                                                      
Jilles Tjoelker