Add default-collect variable.

	Add default-collect variable.
	* tracepoint.c (default_collect): New global.
	(encode_actions): Use it.
	(download_tracepoint): Test it, for otherwise
	action-less tracepoints.
	(_initialize_tracepoint): Add set/show.
	* NEWS: Mention default-collect.

	* gdb.texinfo (Tracepoint Actions): Describe default-collect.

	* gdb.trace/actions.exp: Test default-collect.