Unset SSH_ORIGINAL_COMMAND in forked slave

If we have a SSH_ORIGINAL_COMMAND without a "--slave" in it and fork a
lokal slave which inherits the environment, we run into a fork bomb
because SSH_ORIGINAL_COMMAND will override the slaves argv making it a
master.

Under normal conditions, this can't happen because the
SSH_ORIGINAL_COMMAND from ssh-config is supposed to contain "--slave".
We can only run into that situation with configuration error or a manual
override during debugging.

Anyway, unset SSH_ORIGINAL_COMMAND in the forked slave to avoid the
situation which is difficult to analye.

cmirror.c

@@ -1196,6 +1196,8 @@ static void master(char *master_path, char *slave_user, char *slave, char *slave
                 unlink(unix_socket_name);
         }
 
+        unsetenv("SSH_ORIGINAL_COMMAND");
+
         execvp(args->pdata[0], (char **)args->pdata);
         die("exec %s: %m\n", args->pdata[0]);
     }