linkchecker: Add support for <base> tag
linkchecker: Add support for <base> tag

Add support for the HTML <base> tag as used by mdBook so The Unstable
Book can be checked.

Also cleanup a few things:
* Stop checking the name attribute. It should never have been used and
mdBook has since been fixed not to use it.
* Make sure we only check html files.
* Remove a few unnecessary allocations.

Finally, dead links in The Unstable Book have been fixed.

GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust