The xemacs package was last built for F36. Upstream is dead, so it is not coming back. This PR removes all mention of XEmacs, and the work we used to have to do to support both Emacs and XEmacs. In addition:

• Modernize the Emacs spec file templates
• Describe all current emacs variants
• Recommend building with emacs-nw
• Guard against %{_emacs_version} being undefined; this can happen when koji builds a source RPM
• Update the location of the emacs RPM macro file
• Change British English spelling to American English spelling
• Add semantic line breaks. This PR touches almost every paragraph in the Emacs guidelines anyway.