(gdb-add-pending, gdb-pending-p, gdb-delete-pending): Macros to

(gdb-add-pending, gdb-pending-p, gdb-delete-pending): Macros to
handle pending triggers.
(gdb-threads-mode-map, def-gdb-thread-buffer-command)
(def-gdb-thread-buffer-simple-command)
(gdb-display-stack-for-thread, gdb-display-locals-for-thread)
(gdb-display-registers-for-thread, gdb-frame-stack-for-thread)
(gdb-frame-locals-for-thread, gdb-frame-registers-for-thread): New
commands which show buffers bound to thread.
(gdb-stack-list-locals-regexp): Removed unused regexp.