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.)

testbn.c

@@ -7,6 +7,7 @@
  * testdata/bignum.py.
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
@@ -31,6 +32,8 @@ int random_byte(void)
     return 0;
 }
 
+void logevent(Frontend *frontend, const char *msg) { assert(0); }
+
 #define fromxdigit(c) ( (c)>'9' ? ((c)&0xDF) - 'A' + 10 : (c) - '0' )
 
 /* For Unix in particular, but harmless if this main() is reused elsewhere */