Revert "WebServer: listen on ipv6 if available", as
MHD doesn't support dual-stack sockets.

This reverts commit 1b18aaedda4d3a148c59df6a95b7d0ba21672560.
WebServer: if ipv6 is available, try to start two MHD instances,
one on ipv6, then one on ipv4. On dual-stack systems where listening
on ipv6 also accepts ipv4 connections (at least OSX and Linux), only
one thread will be used and serve both address families.
On others, two threads will be started.