rtapi: Refuse to load uspace components missing rtapi_app_exit

rtapi_app.h mandates that every component exports both rtapi_app_main
and rtapi_app_exit, but do_load_cmd() only checked for main and
do_unload_cmd() silently skipped the exit call when it was absent.
That let enum.c slip into the tree without rtapi_app_exit for years
(Fixes #3191), visible only once lld 17+'s default
--no-undefined-version made it a hard link error.

Check for rtapi_app_exit in do_load_cmd() alongside rtapi_app_main
and refuse to load the component otherwise, so future omissions
surface immediately at load time with a clear error message.

While here, also correct the dlsym cast for rtapi_app_exit in both
do_load_cmd() and do_unload_cmd() from int(*)(void) to void(*)(void)
to match the component's actual declaration. The int version was
copy-pasted from the rtapi_app_main lookup and while harmless in
practice (the return value was never read) it misrepresents the
function signature.

Author: Luca Toniolo

src/rtapi/uspace_rtapi_app.cc

@@ -289,6 +289,12 @@ static int do_load_cmd(const string& name, const vector<string>& args) {
             modules.erase(name);
             return -1;
         }
+        if(!DLSYM<void(*)(void)>(module, "rtapi_app_exit")) {
+            rtapi_print_msg(RTAPI_MSG_ERR, "%s: component is missing rtapi_app_exit\n", name.c_str());
+            dlclose(module);
+            modules.erase(name);
+            return -1;
+        }
         int result;
 
         result = do_comp_args(module, args);
@@ -320,7 +326,7 @@ static int do_unload_cmd(const string& name) {
         rtapi_print_msg(RTAPI_MSG_ERR, "%s: not loaded\n", name.c_str());
 	return -1;
     } else {
-        int (*stop)(void) = DLSYM<int(*)(void)>(w, "rtapi_app_exit");
+        void (*stop)(void) = DLSYM<void(*)(void)>(w, "rtapi_app_exit");
 	if(stop) stop();
 	modules.erase(modules.find(name));
         dlclose(w);