Uppity: allow CLI override of the KEXINIT strings.

This is an obviously useful test feature, since if nothing else it
will let me exercise every individual crypto primitive, even the ones
that the client-side configuration is too coarse-grained to describe
in detail (such as the difference between CBC and CTR mode versions of
the same cipher).

sshserver.h

@@ -1,7 +1,12 @@
 typedef struct AuthPolicy AuthPolicy;
 
 struct SshServerConfig {
-    ptrlen banner;        /* banner.ptr == NULL indicates no banner */
+    /*
+     * In all of these ptrlens, setting the 'ptr' member to NULL means
+     * that we're not overriding the default configuration.
+     */
+    ptrlen banner;                     /* default here is 'no banner' */
+    ptrlen kex_override[NKEXLIST];
 
     bool exit_signal_numeric;          /* mimic an old server bug */
 };