pc_serial: fix gcc4 warnings (hopefully)
Debugger: Implement remaining part of #11033.

TeamDebugInfo:
- Add function to invalidate a source entry's file mapping.
UserInterfaceListener:
- Add listener hook to be able to invoke the aforementioned invalidate.
  Add corresponding implementation in TeamDebugger.
FileManager:
- When asked to locate a source entry, invalidate any existing mapping
  first.
TeamWindow:
- Always allow choosing an alternate source file.
- If we already have an existing source mapping, invalidate it first
  before attempting to establish a new one.
pc_serial: more NULL checks