(gdb-thread-indicator): New variable.

(gdb-thread-indicator): New variable.
(gdb-init-1): Initialise it.
(gdb-annotation-rules): New entry for "new-thread".
(gdb-thread-identification): New function to customize
mode-line-buffer-identification.
(gdb-frames-mode, gdb-registers-mode, gdb-locals-mode)
(gdb-assembler-mode): Use it.
(gdb-threads-mode): Force "info threads" onto queue.