Backends: notify ldisc when sendok becomes true. (NFC)

I've introduced a function ldisc_notify_sendok(), which backends
should call on their ldisc (if they have one) when anything changes
that might cause backend_sendok() to start returning true.

At the moment, the function does nothing. But in future, I'm going to
make ldisc start buffering typed-ahead input data not yet sent to the
backend, and then the effect of this function will be to trigger
flushing all that data into the backend.

Backends only have to call this function if sendok was previously
false: backends requiring no network connection stage (like pty and
serial) can safely return true from sendok, and in that case, they
don't also have to immediately call this function.

putty.h

@@ -2055,6 +2055,7 @@ void ldisc_configure(Ldisc *, Conf *);
 void ldisc_free(Ldisc *);
 void ldisc_send(Ldisc *, const void *buf, int len, bool interactive);
 void ldisc_echoedit_update(Ldisc *);
+void ldisc_check_sendok(Ldisc *);
 
 /*
  * Exports from sshrand.c.