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.

testsc.c

@@ -93,6 +93,10 @@ static NORETURN PRINTF_LIKE(1, 2) void fatal_error(const char *p, ...)
 }
 
 void out_of_memory(void) { fatal_error("out of memory"); }
+FILE *f_open(const Filename *filename, char const *mode, bool is_private)
+{ unreachable("this is a stub needed to link, and should never be called"); }
+void old_keyfile_warning(void)
+{ unreachable("this is a stub needed to link, and should never be called"); }
 
 /*
  * A simple deterministic PRNG, without any of the Fortuna