Simplify XML parsing a bit.

	Simplify XML parsing a bit.

	* xml-support.h (gdb_xml_parse_quick): Declare.
	* xml-support.c (gdb_xml_create_parser_and_cleanup_1): Renamed
	from gdb_xml_create_parser_and_cleanup, and added `old_chain'
	parameter.
	(gdb_xml_create_parser_and_cleanup): Reimplement on top of
	gdb_xml_create_parser_and_cleanup_1.
	(gdb_xml_parse_quick): New.
	* memory-map.c (parse_memory_map): Use gdb_xml_parse_quick.
	* osdata.c (osdata_parse): Ditto.
	* remote.c (remote_threads_info): Ditto.
	* solib-target.c (solib_target_parse_libraries): Ditto.
	* xml-syscall.c (syscall_parse_xml): Ditto.
	* xml-tdesc.c (tdesc_parse_xml): Ditto.