I have heavily customised keybindings on a dvorak layout and this 
necessitates
rebinding quit away from its default value of C-g.

I have rebound C-g in keymaps where it is already defined (such as 
global-map,
minibuffer-local-map and query-replace-map), and have found this to work in
most cases with one major issue in flyspell-mode.

flyspell-post-command-hook calls flyspell-check-word-p, which calls sit-for,
which ultimately calls read_char in keyboard.c.

quit_char in keyboard.c is by default set to ?\C-g and as a consequence in
flyspell-mode if C-g is pressed immediately after entering some text a 
quit is
signalled rather than running the command bound to C-g.

set-quit-char should be able to change quit_char to my desired value of 
?\C-p,
however I run emacs in a graphical window and set-quit-char silently does
nothing if emacs does not have a controlling tty.


To reproduce this issue in emacs 25.0.91.7 with emacs -Q in a graphical
window:

(define-key global-map [?\C-g] 'backward-delete-char)
(define-key global-map [?\C-p] 'keyboard-quit)
(set-quit-char ?\C-p)

;; observe that quit_char is unchanged from original value of ?\C-g
(current-input-mode)


By running flyspell mode, typing something and pressing C-g it can be 
observed
that backward-delete-char is not run as would be expected.

I have attached a patch that changes the behaviour of set-quit-char so that
quit_char is set even if emacs does not have a controlling tty.