dictionary: Avoid deprecation warning for process-kill-without-query
dictionary: Use buffer-read-only variable

This fixes a warning about invalid use of interactive function
toggle-read-only.
dictionary: Replace mapcar with mapc where possible

mapc doesn't return the result, mapcar does. So if the result of the
call is not needed, prefer mapc here.
dictionary: Avoid reference to undefined dictionary-server

Add a defvar so that the setter function for defcustom can set the
variable without triggering a warning.
dictionary: Fix :type for dictionary-tooltip-dictionary

Using type string gives the warning "(mismatch)" when used with the
default value of nil.

Using a more specific type where nil stands for no dictionary and the
alternative string type specifies the dictionary to use.

This was suggested by Kevin Ryde in debian bug #719041.