Work around key algorithm naming change in OpenSSH <= 7.7.

When you send a "publickey" USERAUTH_REQUEST containing a certified
RSA key, and you want to use a SHA-2 based RSA algorithm, modern
OpenSSH expects you to send the algorithm string as
rsa-sha2-NNN-cert-v01@openssh.com. But 7.7 and earlier didn't
recognise those names, and expected the algorithm string in the
userauth request packet to be ssh-rsa-cert-v01@... and would then
follow it with an rsa-sha2-NNN signature.

OpenSSH itself has a bug workaround for its own older versions. Follow
suit.

ssh.h

@@ -1882,6 +1882,7 @@ void old_keyfile_warning(void);
     X(BUG_SENDS_LATE_REQUEST_REPLY)             \
     X(BUG_SSH2_OLDGEX)                          \
     X(BUG_REQUIRES_FILTERED_KEXINIT)            \
+    X(BUG_RSA_SHA2_CERT_USERAUTH)               \
     /* end of list */
 #define TMP_DECLARE_LOG2_ENUM(thing) log2_##thing,
 enum { SSH_IMPL_BUG_LIST(TMP_DECLARE_LOG2_ENUM) };