Add comioDrainOutput() function. Hopefully this will allow the dtr_delay

thing to go away if Win32 has something similar.

src/comio/comio_nix.c

@@ -205,3 +205,7 @@ BOOL comPurgeOutput(COM_HANDLE handle)
 	return(tcflush(handle, TCOFLUSH)==0);
 }
 
+BOOL comDrainOutput(COM_HANDLE handle)
+{
+	return(tcdrain(handle)==0);
+}