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.

utils.c

@@ -1049,3 +1049,27 @@ size_t encode_utf8(void *output, unsigned long ch)
     }
     return p - start;
 }
+
+void write_c_string_literal(FILE *fp, ptrlen str)
+{
+    for (const char *p = str.ptr; p < (const char *)str.ptr + str.len; p++) {
+        char c = *p;
+
+        if (c == '\n')
+            fputs("\\n", fp);
+        else if (c == '\r')
+            fputs("\\r", fp);
+        else if (c == '\t')
+            fputs("\\t", fp);
+        else if (c == '\b')
+            fputs("\\b", fp);
+        else if (c == '\\')
+            fputs("\\\\", fp);
+        else if (c == '"')
+            fputs("\\\"", fp);
+        else if (c >= 32 && c <= 126)
+            fputc(c, fp);
+        else
+            fprintf(fp, "\\%03o", (unsigned char)c);
+    }
+}