Make template_header available to recipes

libraries/template_header.rb directly monkey-patches Chef::Mixin::Template::TemplateContext.

Instead it should define a Sys::TemplateHeader module that can then be included in Chef::Mixin::Template::TemplateContext.

This would allow a recipe to:

Chef::Recipe.include(Sys::TemplateHeader)

file 'bla' do
  content <<-CONTENT
    #{template_header}
    …
  CONTENT
  …
end