Draft: httpd server implementation

Current status:

• MHD module draft: A host is created and started upon module load. Inside the module, custom number of worker threads are provided via --http-threads flag (defaults to 1).
• vlc_httpd_* APIs draft

Pending plan:

• For the MHD module, the implementation of gnuTLS and user/password authentication is still pending
• In the core, I plan to provide high-level APIs to register files, handlers, redirects, and streams, similar to the legacy module. These will serve as abstractions of the basic module APIs.
• Testing the entire stack with a client module is pending