samples: net: Add a multithreaded dumb HTTP server

This adds a multithreaded dumb HTTP server sample application similar
to dumb_http_server with following differences

* support both IPv4 and IPv6 simultaneously
* support multiple incoming connections
* support TLS connections
* ignore POSIX compatibility

This app can be used for testing. Note that the app will always
serve a same static HTML page to caller regardless of HTTP request
parameters.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>