cmdgen: add a --dump option.

Also spelled '-O text', this takes a public or private key as input,
and produces on standard output a dump of all the actual numbers
involved in the key: the exponent and modulus for RSA, the p,q,g,y
parameters for DSA, the affine x and y coordinates of the public
elliptic curve point for ECC keys, and all the extra bits and pieces
in the private keys too.

Partly I expect this to be useful to me for debugging: I've had to
paste key files a few too many times through base64 decoders and hex
dump tools, then manually decode SSH marshalling and paste the result
into the Python REPL to get an integer object. Now I should be able to
get _straight_ to text I can paste into Python.

But also, it's a way that other applications can use the key
generator: if you need to generate, say, an RSA key in some format I
don't support (I've recently heard of an XML-based one, for example),
then you can run 'puttygen -t rsa --dump' and have it print the
elements of a freshly generated keypair on standard output, and then
all you have to do is understand the output format.

sshpubk.c

@@ -1730,3 +1730,47 @@ const char *key_type_to_str(int type)
         unreachable("bad key type in key_type_to_str");
     }
 }
+
+key_components *key_components_new(void)
+{
+    key_components *kc = snew(key_components);
+    kc->ncomponents = 0;
+    kc->componentsize = 0;
+    kc->components = NULL;
+    return kc;
+}
+
+void key_components_add_text(key_components *kc,
+                             const char *name, const char *value)
+{
+    sgrowarray(kc->components, kc->componentsize, kc->ncomponents);
+    size_t n = kc->ncomponents++;
+    kc->components[n].name = dupstr(name);
+    kc->components[n].is_mp_int = false;
+    kc->components[n].text = dupstr(value);
+}
+
+void key_components_add_mp(key_components *kc,
+                           const char *name, mp_int *value)
+{
+    sgrowarray(kc->components, kc->componentsize, kc->ncomponents);
+    size_t n = kc->ncomponents++;
+    kc->components[n].name = dupstr(name);
+    kc->components[n].is_mp_int = true;
+    kc->components[n].mp = mp_copy(value);
+}
+
+void key_components_free(key_components *kc)
+{
+    for (size_t i = 0; i < kc->ncomponents; i++) {
+        sfree(kc->components[i].name);
+        if (kc->components[i].is_mp_int) {
+            mp_free(kc->components[i].mp);
+        } else {
+            smemclr(kc->components[i].text, strlen(kc->components[i].text));
+            sfree(kc->components[i].text);
+        }
+    }
+    sfree(kc->components);
+    sfree(kc);
+}