webif: Allow webif to start when IPv6 is enabled but unsupported.

The logic for falling back to IPv4 if IPv6 socket creation failed
was not working, which prevented webif from starting when IPv6 support
was enabled in the build but the kernel didn't support IPv6.

This fixes ticket #3348.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8692 4b0bc96b-bc66-0410-9d44-ebda105a78c1