Reorder proxy-type enum for backwards compatibility.

Commit 0d3bb73608f1a5d inserted PROXY_SSH just before PROXY_CMD, so
that it got the numerical value that PROXY_CMD had before. But that
meant that any saved session configured as PROXY_CMD by an older build
of PuTTY would be regarded as PROXY_SSH by the new version - even
after the previous commit fixed the unconditional use of SSH proxying
regardless of the type setting.

Now PROXY_CMD has its old value back, and PROXY_SSH is inserted at the
_end_ of the enum. (Or rather, just before the extra-weird PROXY_FUZZ,
which is allowed to have an unstable numerical value because it's
never stored in a saved session at all.)

This is all rather unsatisfactory, and makes me wish I'd got round to
reworking the saved data format to use keywords in place of integers.

putty.h

@@ -474,8 +474,8 @@ enum {
      * Proxy types.
      */
     PROXY_NONE, PROXY_SOCKS4, PROXY_SOCKS5,
-    PROXY_HTTP, PROXY_TELNET, PROXY_SSH,
+    PROXY_HTTP, PROXY_TELNET, PROXY_CMD, PROXY_SSH,
-    PROXY_CMD, PROXY_FUZZ
+    PROXY_FUZZ
 };
 
 enum {