Implement -list-thread-groups --available

	Implement -list-thread-groups --available

        * Makefile.in (XMLFILES): Add osdata.dtd.
        (SFILES): Add osdata.c.
        (COMMON_OBS): Add osdata.o.
        * linux-nat.c: Include pwd.h, sys/types.h, gdb_dirent.h and xml-support.h.
        (linux_nat_xfer_osdata): New function.
        (linux_xfer_partial): Handle TARGET_OBJECT_OSDATA.
        * osdata.c: New file.
        * osdata.h: New file.
        * remote.c (PACKET_qXfer_osdata): New packet enum.
        (remote_protocol_features): Add "qXfer:osdata:read".
        (remote_read_qxfer): Handle TARGET_OBJECT_OSDATA.
        (extended_remote_can_run): New.
        (init_extended_remote_ops): Set to_can_run to
        extended_remote_can_run.
        (_initialize_remote): Add packet config command for
        "qXfer:osdata:read".
        * xml-support.c (obstack_xml_printf): New function.
        * xml-support.h (obstack_xml_printf): Declare.
        * target.c (target_get_osdata): New function.
        * target.h (enum target_object): Add TARGET_OBJECT_OSDATA.
        (target_os_data): Declare.
        * features/osdata.dtd: New file.
        * mi/mi-main.c (mi_list_thread_groups): Handle the --available
        option.