Blog URLs should not require hostname

[jergendutch]

Wordpress 2.5.1 (and probably other versions) requires a hostname in the Blog URL configuration field.

Wordpress should behave gracefully if the hostname is missing by substituting $_SERVER[SERVER_NAME] instead.

[DD32]

See #6890, I'm pretty sure this is basically a duplicate, but not strictly sure as this could be referencing other uses of urls around the place.

[jergendutch]

Let's mark it as a duplicate.