bug#25708: 25.1.91; Allow users to inhibit printing for 'emacsclient -c -t'

[Alex Hutcheson]

Sometimes I want to eval some elisp at the command line exclusively to
set up some buffers for me to work with in a new frame. For example, to
diff two files, I often do:
  emacsclient -c -e '(ediff "fileA" "fileB")

This works, but emacsclient annoyingly prints the result of the eval'ed
expression to stdout. In this case, I'll get:
  #<buffer *Ediff Control Panel*>

This output is annoying, and it can't be redirected (`emacsclient -c' on a
terminal fail to launch if stdout is redirected, for obvious reasons).

It would be great if emacsclient had a way to inhibit printing the
result, for cases where the user is only eval'ing the expressions for
their side effects.