ENHANCED: handle renaming variables to existing variable names

ENHANCED: handle renaming variables to existing variable names

* sweeprolog.el (sweeprolog-rename-variable-allow-existing): new user
option, used by...
(sweeprolog-read-new-variable-try): new command, exists the minibuffer
if it contains a valid variable name.  It checks if the selected
variable name already exists.  Used by...
(sweeprolog-read-new-variable): new function, reads a Prolog variable
name in the minibuffer with a dedicated exit command.
(sweeprolog-rename-variable): use it.