re-enable C-x C-c, make it work

When I first switched to emacs server and emacsclient, I kept hitting
C-x C-c and killing my emacs session.  So I remapped it to remind me not
to do that.  That was many years ago, and my fingers no longer jump to
C-x C-c when I'm ready to close a buffer.

Also made this work better when using 'emacsclient -t', see comments.