Add wasm-c-api nested function calls sample (#652)

And enable to copy back the return value of wasm main function when calling wasm_application_execute_main, add license headers in wasm-c-api samples, fix several issues reported by klocwork.

core/iwasm/common/wasm_application.c

@@ -182,6 +182,10 @@ wasm_application_execute_main(WASMModuleInstanceCommon *module_inst,
 
     ret = wasm_runtime_create_exec_env_and_call_wasm(module_inst, func,
                                                      argc1, argv1);
+    if (ret && func_type->result_count > 0 && argc > 0 && argv)
+        /* copy the return value */
+        *(int*)argv = (int)argv1[0];
+
     if (argv_buf_offset)
         wasm_runtime_module_free(module_inst, argv_buf_offset);
     return ret;
@@ -669,4 +673,4 @@ fail:
     bh_assert(exception);
     os_printf("%s\n", exception);
     return false;
-}
+}

core/iwasm/include/wasm_export.h

@@ -495,7 +495,9 @@ wasm_runtime_call_wasm_v(wasm_exec_env_t exec_env,
  *
  * @param module_inst the WASM module instance
  * @param argc the number of arguments
- * @param argv the arguments array
+ * @param argv the arguments array, if the main function has return value,
+ *   *(int*)argv stores the return value of the called main function after
+ *   this function returns.
  *
  * @return true if the main function is called, false otherwise and exception
  *   will be thrown, the caller can call wasm_runtime_get_exception to get