trivial: Fix compiler warnings

- Add missing return statements
- Add missing const modifiers
- Initialize variables correctly
- Remove unused variables
- Correct incorrect type definitions
- Use correct printf format string types

GPIOMUXServer: Refactor client-server interface

Use the connector definitions to control interface configuration and
initialization.

Provide an ADL API for creating the GPIOServer client-server interface
in any component.

Provide an ADL API for creating the MUXServer client-server interface in
any component.