Implement part of `ssh2-generality': introduce the ability to tell

PuTTY / Plink not to run a remote shell/command at all. Supported in
the GUI configuration and via the (OpenSSH-like) -N command-line
option.

No effort is currently made to arrange `nice' UI properties. If you
do this in GUI PuTTY, a full-size terminal window will still be
created, and will sit there with almost nothing in it throughout
your session. If you do it in Plink, Plink will not accept any kind
of request to terminate gracefully; you'll have to ^C or kill it.
Nonetheless, even this little will be useful to some people...

[originally from svn r4614]

putty.h

@@ -358,6 +358,7 @@ struct config_tag {
     int try_ki_auth;
     int ssh_subsys;		       /* run a subsystem rather than a command */
     int ssh_subsys2;		       /* fallback to go with remote_cmd2 */
+    int ssh_no_shell;		       /* avoid running a shell */
     /* Telnet options */
     char termtype[32];
     char termspeed[32];