When emacsclient is used in a terminal, emacsclient FILE opens a 
non-graphical Emacs frame, and emacsclient -c FILE always creates a new 
graphical Emacs frame.  A third option, which creates a graphical frame if 
none exist and reuses a graphical frame is one exists, would be useful 
(see for example [1] and [2]).

Patch attached.

[1] https://superuser.com/questions/358037/emacsclient-create-a-frame-if-a-frame-does-not-exist 
[2] https://emacs.stackexchange.com/questions/12894/make-emacsclient-create-a-frame-only-if-there-isnt-one-already