[css21] 'font-family' should disallow 'inherit' in font names more strictly

Right now, 2.1 says the following about using 'inherit' in a font name:

# The keywords 'initial' and 'default' are reserved for
# future use and must also be quoted when used as
# font names.

This isn't strict enough, and implementations do somewhat different
things around this.  See Kenny's email at
<http://lists.w3.org/Archives/Public/www-style/2012Apr/0080.html>
where he shows several differences in implementations, where some UAs
accept 'inherit' as part of a font-name, or even as an entire
font-name in some circumstances.

We propose tightening this up, since there's no use-case for using
unquoted 'inherit' in a font name, to:

> The keywords 'initial' and 'default' are reserved for
> future use and must also be quoted when used _in_
> font names.

~TJ and fantasai