Replace uses of (gdb-get-instance-buffer 'gdba) and

Replace uses of (gdb-get-instance-buffer 'gdba) and
gdb-proc with gud-comint-buffer instead.
(gdb-proc): Remove.
(def-gdb-var): Simplify.
(gdb-get-create-instance-buffer): Avoid make-variable-buffer-local.
Set gud-comint-buffer as well as gud-minor-mode and tool-bar-map.
(gdb-goto-bp-this-line): Select the window while doing the goto-line.
(gdb-info-locals-handler): Avoid replace-regexp.