[win32] bump libmicrohttpd to 0.9.17

After libmicrohttpd has been bumped from 0.4.6 to 0.9.x for darwin (not sure about linux?) I've taken a look into updating it for win32 as well. I went for the 0.9.17 release because it's the newest release with pre-built win32 binaries.

The first commit contains some general cleanup concerning the webserver. The second commit cleans up the includes used by microhttpd.h. The third commit changes the loaded libmicrohttpd DLL (the dependency file hasn't been uploaded to our mirrors yet).

I didn't do any extensive testing except starting and stopping the webserver a few times, playing around with the webinterface and making some json-rpc requests over HTTP but I didn't notice anything out of the ordinary.

PS: This bump increases the size of the win32 install exe by approximately 2 MB because of the included HTTPS/TLS support.