Implement my experimental arcfour modes. The 256-bit version is disabled

until I can test it against someone else's implementation.

[originally from svn r5633]

doc/config.but

@@ -2127,6 +2127,8 @@ PuTTY currently supports the following algorithms:
 
 \b \i{AES} (Rijndael) - 256, 192, or 128-bit CBC (SSH-2 only)
 
+\b \i{Arcfour} (RC4) - 256 or 128-bit stream cipher (SSH-2 only)
+
 \b \i{Blowfish} - 128-bit CBC
 
 \b \ii{Triple-DES} - 168-bit CBC

doc/errors.but

@@ -74,7 +74,7 @@ this, let us know and we'll move it up our priority list.
 
 This occurs when the SSH server does not offer any ciphers which you
 have configured PuTTY to consider strong enough. By default, PuTTY
-puts up this warning only for \ii{single-DES} encryption.
+puts up this warning only for \ii{single-DES} and \i{Arcfour} encryption.
 
 See \k{config-ssh-encryption} for more information on this message.
 

doc/index.but

@@ -595,6 +595,9 @@ saved sessions from
 \IM{AES} Advanced Encryption Standard
 \IM{AES} Rijndael
 
+\IM{Arcfour} Arcfour
+\IM{Arcfour} RC4
+
 \IM{triple-DES} triple-DES
 
 \IM{single-DES} single-DES