(gdb-init-1): Move sending

(gdb-init-1): Move sending
-data-list-register-names to ...
(gdb-starting): ... here because GDB 7.0 requires execution to
have started when using this MI command.
(gdb-set-header): New function to distinguish select and
unselected tabs in gdb buffers.
(gdb-propertize-header): New macro that uses gdb-set-header.
(gdb-breakpoints-header, gdb-locals-header): Use it.