New utility function logevent_and_free.

This should make it easier to do formatted-string based logging
outside ssh.c, because I can wrap up a local macro in any source file
I like that expands to logevent_and_free(wherever my Frontend is,
dupprintf(macro argument)).

It caused yet another stub function to be needed in testbn, but there
we go.

(Also, while I'm here, removed a redundant declaration of logevent
itself from ssh.h. The one in putty.h is all we need.)

misc.c

@@ -337,6 +337,12 @@ void burnstr(char *string)             /* sfree(str), only clear it first */
     }
 }
 
+void logevent_and_free(Frontend *frontend, char *s)
+{
+    logevent(frontend, s);
+    sfree(s);
+}
+
 int toint(unsigned u)
 {
     /*