(gdb-breakpoints-mode-map): Don't assume threads buffer is present.

(gdb-breakpoints-mode-map): Don't assume threads buffer is present.
(gdb-threads-mode-map): Don't assume breakpoints buffer is present.
(gdb-disassembly-handler-custom, gdb-stack-list-frames-custom)
(gdb-locals-handler-custom, gdb-registers-handler-custom): Thread
info in mode name.
(gdb-registers-mode-map): TAB to switch to locals.