(gdb-thread-number): New variable.

(gdb-thread-number): New variable.
(gdb-current-context-command): New macro which adds --thread
option to command.
(gdb-threads-mode-map): Select thread with SPC
(gdb-thread-list-handler-custom): Mark current thread with overlay
arrow. Synchronize GDB thread and Emacs thread.
(gdb-select-thread): New command which selects current thread.
(gdb-invalidate-frames, gdb-invalidate-locals)
(gdb-invalidate-registers): Use --thread option.