(gdb-signalled): New variable and function.

(gdb-signalled): New variable and function.
(gdb-debug-log-ring): Rename from gdb-debug-log and make a ring.
(gdb-send, gdb-send-item, gud-gdba-marker-filter): Use it.
(gdb-debug-log-length): Customize it's length.
(gud-watch, gdb-var-create-handler): Display function::var format
but don't use to create variable object.
(gdb-var-create-handler): Use message-box in place of message.
(gdb-stopped): Call gdb-exited if signalled.