Add support for thin archives.

Add support for thin archives.
    * bfd/archive.c (_bfd_find_nested_archive): New function.
    (get_extended_arelt_filename): Add origin parameter.
    (_bfd_generic_read_ar_hdr_mag): Deal with extended name
    combined with a file offset.
    (append_relative_path): New function.
    (_bfd_get_elt_at_filepos): Deal with external members and
    nested archives.
    (bfd_generic_openr_next_archived_file): Thin archives.
    (bfd_generic_archive_p): Recognize new magic string.
    (adjust_relative_path): New function.
    (_bfd_construct_extended_name_table): Construct extended
    names for thin archive members.
    (_bfd_write_archive_contents): Emit new magic string, skip
    copying files for thin archives.
    * bfd/bfd-in.h (bfd_is_thin_archive): New macro.
    * bfd/bfd.c (struct bfd): New fields for thin archives.
    * bfd/libbfd-in.h (struct areltdata): New field for thin archives.
    * bfd/opncls.c (bfd_close): Delete BFDs for nested archives.
    * binutils/ar.c (make_thin_archive): New global flag.
    (map_over_members): Deal with full pathnames in thin archives.
    (usage, main): Add 'T' option for building thin archives.
    (replace_members): Pass thin archive flag to ar_emul_append.
    * binutils/arsup.c (ar_open): Initialize new flag.
    * binutils/binemul.c (ar_emul_append): Add new parameter for
    flattening nested archives.
    (do_ar_emul_default_append): New function.
    (ar_emul_default_append): Factored out recursive code.
    * binutils/binemul.h (ar_emul_default_append): Add new parameter.
    (struct bin_emulation_xfer_struct): New parameter for ar_append.
    * binutils/dlltool.c (gen_lib_file): Initialize thin archive flag.
    * binutils/emul_aix.c (ar_emul_aix_internal): Add new flatten
    parameter, currently unimplemented.
    All callers changed.
    * binutils/objcopy.c (copy_archive): Preserve thin archive flag.
    * binutils/doc/binutils.texi: Update ar documentation.
    * binutils/testsuite/binutils-all/ar.exp: Add thin archive tests.
    * include/aout/ar.h (ARMAGT): New magic string for thin archives.