Fix handling double-click-time nil or t

Fix handling double-click-time nil or t

* lisp/mouse.el (mouse-double-click-time): New function to always
return a number for `double-click-time'.
* lisp/emulation/viper-mous.el (viper-multiclick-timeout):
* lisp/foldout.el (foldout-mouse-swallow-events):
* lisp/help.el (help--read-key-sequence):
* lisp/org/org-mouse.el (org-mouse-show-context-menu): Use
'mouse-double-click-time' instead of 'double-click-time'.
* src/keyboard.c (syms_of_keyboard): Mention
'mouse-double-click-time' in doc string of 'double-click-time'.
* test/lisp/mouse-tests.el (mouse-test-mouse-double-click-time):
New test.