'multisite' should be renamed to 'multisite.xml'

[scribu]

I would like to challenge [926] and suggest that multisite.xml makes more sense as the name for the XML configuration file.

Additionally, I think phpunit.xml should be renamed to singlesite.xml.

Nevermind renaming phpunit.xml. Doing so would mean that it would have to be explicitly passed as a parameter.

But having a file that's called 'multisite' is just confusing; is it a binary? Am I on Windows and the extension is hidden?

Also, you get no syntax highlighting when editing it.

Related: #112

+1, was about to report this myself finally

(In [1023]) Rename multisite configuration file to 'multisite.xml'. Fixes #111