Windows PuTTYgen: support cmdgen's key-type and PPK options.

This imports the following options from command-line PuTTYgen, which
all correspond to controls in Windows PuTTYgen's GUI, and let you set
the GUI controls to initial values of your choice:

  -t <key type>
  -b <bits>
  -E <fingerprint type>
  --primes <prime gen policy>
  --strong-rsa
  --ppk-param <KDF parameters or PPK version etc>

The idea is that if someone generates a lot of keys and has standard
non-default preferences, they can make a shortcut that passes those
preferences on the command line.

doc/pubkey.but

@@ -478,6 +478,82 @@ you have generated an SSH-1 private key using OpenSSH or
 Hence, the export options are not available if you have generated an
 SSH-1 key.
 
+\S{puttygen-cli} PuTTYgen command-line configuration
+
+PuTTYgen supports a set of command-line options to configure many of
+the same settings you can select in the GUI. This allows you to start
+it up with your own preferences ready-selected, which might be useful
+if you generate a lot of keys. (For example, you could make a Windows
+shortcut that runs PuTTYgen with some command line options, or a batch
+file or Powershell script that you could distribute to a whole
+organisation containing your local standards.)
+
+The options supported on the command line are:
+
+\dt \cw{\-t} \e{keytype}
+
+\dd Type of key to generate. You can select \c{rsa}, \c{dsa},
+\c{ecdsa}, \c{eddsa} or \c{rsa1}. See \k{puttygen-keytype}.
+
+\dt \cw{\-b} \e{bits}
+
+\dd Size of the key to generate, in bits. See \k{puttygen-strength}.
+
+\dt \cw{\-\-primes} \e{method}
+
+\dd Method for generating prime numbers. You can select \c{probable},
+\c{proven}, and \c{proven-even}. See \k{puttygen-primes}.
+
+\dt \cw{\-\-strong-rsa}
+
+\dd When generating an RSA key, make sure the prime factors of the key
+modulus are \q{strong primes}. A strong prime is a prime number chosen
+to have a particular structure that makes certain factoring algorithms
+more difficult to apply, so some security standards recommend their
+use. However, the most modern factoring algorithms are unaffected, so
+this option is probably not worth turning on \e{unless} you have a
+local standard that recommends it.
+
+\dt \cw{\-\-ppk-param} \e{key}\cw{=}\e{value}\cw{,}...
+
+\dd Allows setting all the same details of the PPK save file format
+described in \k{puttygen-save-params}.
+
+\lcont{
+
+Aspects to change are specified as a series of \e{key}\cw{=}\e{value} pairs
+separated by commas. The \e{key}s are:
+
+\dt \cw{version}
+
+\dd The PPK format version: either \cw{3} or \cw{2}.
+
+\dt \cw{kdf}
+
+\dd The variant of Argon2 to use: \cw{argon2id}, \cw{argon2i}, and
+\cw{argon2d}.
+
+\dt \cw{memory}
+
+\dd The amount of memory needed to decrypt the key, in Kbyte.
+
+\dt \cw{time}
+
+\dd Specifies how much time is required to attempt decrypting the key,
+in milliseconds.
+
+\dt \cw{passes}
+
+\dd Alternative to \cw{time}: specifies the number of hash passes
+required to attempt decrypting the key.
+
+\dt \cw{parallelism}
+
+\dd Number of parallelisable threads that can be used to decrypt the
+key.
+
+}
+
 \H{pubkey-gettingready} Getting ready for public key authentication
 
 Connect to your SSH server using PuTTY with the SSH protocol. When the