Move 'loaded_session' into cmdline.c.

I haven't managed to make this one _not_ be a mutable variable, but at
least it's not global across all tools any more: it lives in cmdline.c
along with the code that decides what to set it to, and cmdline.c
exports a query method to ask for its value.

windows/winplink.c

@@ -260,7 +260,6 @@ int main(int argc, char **argv)
      */
     conf = conf_new();
     do_defaults(NULL, conf);
-    loaded_session = false;
     default_protocol = conf_get_int(conf, CONF_protocol);
     default_port = conf_get_int(conf, CONF_port);
     errors = false;