* doc/emacs/misc.texi: (Using Emacs as a Server) Explain
emacsclient.desktop.
* etc/NEWS: (Emacs Server): Explain emacsclient.desktop.
* emacs-mail.desktop, etc/emacsclient.desktop: Automatically try to
reuse an existing frame, open a new frame, or start a new Emacs
daemon. Add actions for specific behaviours (bug#49195).
* etc/emacsclient.desktop: Undo setting of StartupWMClass=Emacsd, since
this relies on a change to etc/emacs.service which was also undone. See
bug#37847 for more explanation (bug#49259).
* Makefile.in: Add emacsclient.desktop generation.
* etc/emacsclient.desktop: Add file, use emacsd as StartupWMClass.
* etc/emacs.service: Run with name emacsd (bug#37847).