Uppity: option to use a pregenerated key for RSA kex.

As and when I make this SSH server into a test suite, I'm not going to
want to wait for a gratuitous RSA key generation in every test run. So
now you can provide one in advance.

It has to be in SSH-1 format, because that's the format for which I
happen to already have internal API routines that return an RSAKey
instead of an opaque ssh_key. But since you also have to store it
without a passphrase, that doesn't really matter anyway.

sshserver.h

@@ -1,6 +1,8 @@
 typedef struct AuthPolicy AuthPolicy;
 
 struct SshServerConfig {
+    RSAKey *rsa_kex_key;
+
     /*
      * In all of these ptrlens, setting the 'ptr' member to NULL means
      * that we're not overriding the default configuration.