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.

ssh.h

@@ -397,6 +397,7 @@ LogContext *ssh_get_logctx(Ssh *ssh);
 void ssh_throttle_conn(Ssh *ssh, int adjust);
 void ssh_got_exitcode(Ssh *ssh, int status);
 void ssh_ldisc_update(Ssh *ssh);
+void ssh_check_sendok(Ssh *ssh);
 void ssh_got_fallback_cmd(Ssh *ssh);
 bool ssh_is_bare(Ssh *ssh);