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.

testcrypt.h

@@ -157,8 +157,17 @@ FUNC2(void, ssh_key_public_blob, val_key, out_val_string_binarysink)
 FUNC2(void, ssh_key_private_blob, val_key, out_val_string_binarysink)
 FUNC2(void, ssh_key_openssh_blob, val_key, out_val_string_binarysink)
 FUNC1(val_string_asciz, ssh_key_cache_str, val_key)
+FUNC1(val_keycomponents, ssh_key_components, val_key)
 FUNC2(uint, ssh_key_public_bits, keyalg, val_string_ptrlen)
 
+/*
+ * Accessors to retrieve the innards of a 'key_components'.
+ */
+FUNC1(uint, key_components_count, val_keycomponents)
+FUNC2(opt_val_string_asciz_const, key_components_nth_name, val_keycomponents, uint)
+FUNC2(opt_val_string_asciz_const, key_components_nth_str, val_keycomponents, uint)
+FUNC2(opt_val_mpint, key_components_nth_mp, val_keycomponents, uint)
+
 /*
  * The ssh_cipher abstraction. The in-place encrypt and decrypt
  * functions are wrapped to replace them with versions that take one