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.h

@@ -31,6 +31,13 @@ char *dupprintf(const char *fmt, ...)
 char *dupvprintf(const char *fmt, va_list ap);
 void burnstr(char *string);
 
+/*
+ * Pass a dynamically allocated string to logevent and immediately
+ * free it. Intended for use by wrapper macros which pass the return
+ * value of dupprintf straight to this.
+ */
+void logevent_and_free(Frontend *frontend, char *msg);
+
 struct strbuf {
     char *s;
     unsigned char *u;