rustdoc: replace DIV inside H1 with SPAN.

W3C HTML5 spec states that H1 must enclose "phrasing content" which
doesn't include DIV.  But SPAN is OK.
http://www.w3.org/TR/html5/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
auto merge of #17529 : nodakai/rust/rustdoc-div-in-h1, r=alexcrichton

W3C HTML5 spec states that H1 must enclose "phrasing content" which
doesn't include DIV.  But SPAN is OK.
http://www.w3.org/TR/html5/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
Convert cfg syntax to new system

This removes the ability to use `foo(bar)` style cfgs. Switch them to
`foo_bar` or `foo="bar"` instead.

[breaking-change]
auto merge of #17527 : sfackler/rust/cfg-syntax, r=alexcrichton

We'll need a snapshot before we can convert the codebase over and turn on the deprecation warnings.

cc #17490

This is sitting on top of #17506