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.)

windows/winpgen.c

@@ -60,6 +60,9 @@ void nonfatal(const char *fmt, ...)
     sfree(stuff);
 }
 
+/* Stub needed to link against misc.c */
+void logevent(Frontend *frontend, const char *msg) { assert(0); }
+
 /* ----------------------------------------------------------------------
  * Progress report code. This is really horrible :-)
  */

windows/winpgnt.c

@@ -113,6 +113,9 @@ static void unmungestr(char *in, char *out, int outlen)
     return;
 }
 
+/* Stub needed to link against misc.c */
+void logevent(Frontend *frontend, const char *msg) { assert(0); }
+
 static int has_security;
 
 struct PassphraseProcStruct {