New bug workaround: KEXINIT filtering.

We've occasionally had reports of SSH servers disconnecting as soon as
they receive PuTTY's KEXINIT. I think all such reports have involved
the kind of simple ROM-based SSH server software you find in small
embedded devices.

I've never been able to prove it, but I've always suspected that one
possible cause of this is simply that PuTTY's KEXINIT is _too long_,
either in number of algorithms listed or in total length (especially
given all the ones that end in @very.long.domain.name suffixes).

If I'm right about either of those being the cause, then it's just
become even more likely to happen, because of all the extra
Diffie-Hellman groups and GSSAPI algorithms we just threw into our
already-long list in the previous few commits.

A workaround I've had in mind for ages is to wait for the server's
KEXINIT, and then filter our own down to just the algorithms the
server also mentioned. Then our KEXINIT is no longer than that of the
server, and hence, presumably fits in whatever buffer it has. So I've
implemented that workaround, in anticipation of it being needed in the
near future.

(Well ... it's not _quite_ true that our KEXINIT is at most the same
length as the server. In fact I had to leave in one KEXINIT item that
won't match anything in the server's list, namely "ext-info-c" which
gates access to SHA-2 based RSA. So if we turn out to support
absolutely everything on all the server's lists, then our KEXINIT
would be a few bytes longer than the server's, even with this
workaround. But that would only cause trouble if the server's outgoing
KEXINIT was skating very close to whatever buffer size it has for the
incoming one, and I'm guessing that's not very likely.)

((Another possible cause of this kind of disconnection would be a
server that simply objects to seeing any KEXINIT string it doesn't
know how to speak. But _surely_ no such server would have survived
initial testing against any full-featured client at all!))

windows/help.h

@@ -170,6 +170,7 @@ typedef const char *HelpCtx;
 #define WINHELP_CTX_ssh_bugs_chanreq "config-ssh-bug-chanreq"
 #define WINHELP_CTX_ssh_bugs_oldgex2 "config-ssh-bug-oldgex2"
 #define WINHELP_CTX_ssh_bugs_dropstart "config-ssh-bug-dropstart"
+#define WINHELP_CTX_ssh_bugs_filter_kexinit "config-ssh-bug-filter-kexinit"
 #define WINHELP_CTX_serial_line "config-serial-line"
 #define WINHELP_CTX_serial_speed "config-serial-speed"
 #define WINHELP_CTX_serial_databits "config-serial-databits"