Move echo/edit state change functionality out of ldisc_send.

I'm not actually sure why we've always had back ends notify ldisc of
changes to echo/edit settings by giving ldisc_send(ldisc,NULL,0,0) a
special meaning, instead of by having a separate dedicated notify
function with its own prototype and parameter set. Coverity's recent
observation that the two kinds of call don't even have the same
requirements on the ldisc (particularly, whether ldisc->term can be
NULL) makes me realise that it's really high time I separated the two
conceptually different operations into actually different functions.

While I'm here, I've renamed the confusing ldisc_update() function
which that special operation ends up feeding to, because it's not
actually a function applying to an ldisc - it applies to a front end.
So ldisc_send(ldisc,NULL,0,0) is now ldisc_echoedit_update(ldisc), and
that in turn figures out the current echo/edit settings before passing
them on to frontend_echoedit_update(). I think that should be clearer.

pscp.c

@@ -60,16 +60,7 @@ const char *const appname = "PSCP";
  */
 #define MAX_SCP_BUFSIZE 16384
 
-void ldisc_send(void *handle, char *buf, int len, int interactive)
-{
-    /*
-     * This is only here because of the calls to ldisc_send(NULL,
-     * 0) in ssh.c. Nothing in PSCP actually needs to use the ldisc
-     * as an ldisc. So if we get called with any real data, I want
-     * to know about it.
-     */
-    assert(len == 0);
-}
+void ldisc_echoedit_update(void *handle) { }
 
 static void tell_char(FILE * stream, char c)
 {