* lisp/progmodes/gdb-mi.el: Minor style changes.

* lisp/progmodes/gdb-mi.el: Minor style changes.
(gdb-enable-debug, gdb-speedbar-auto-raise, gdb-many-windows):
Turn into minor modes.
(gdb--if-arrow): Rename from gdb-if-arrow, make it hygienic.
(gdb-mouse-until, gdb-mouse-jump): Adjust uses accordingly.
(gdb-shell): Remove unneeded let-binding.
(gdb-get-many-fields): Eliminate O(n²) behavior.