webserver: improved caching control

The goal of this PR is to improve the caching functionality and control of clients/webbrowsers interacting with our webserver. The first (2c2b7f5) and last (f927927) commit contain pure refactoring to adjust the code in CWebServer to match our coding guidelines. The second to last (072306a) commit silences a simple warning. The other commits add the following functionalities:
* 19dcc76 adds handling of `If-Unmodified-Since` (we already handle `If-Modified-Since`) in HTTP requests
* 7b5816d adds `Cache-Control: max-age=<x> public` to any HTTP responses that contain the content of a file. This extends the existing `Expires` functionality from HTTP/1.0 with the recommended `Cache-Control` functionality from HTTP/1.1
* aff2a39 adds handling of `Cache-Control: no-cache` from HTTP/1.1 and of the deprecated `Pragma: no-cache` in HTTP request
* fa808c3 just changes all hard-coded strings for HTTP header fields and values to custom defines and defines from libmicrohttpd to avoid typos