(xheader_keyword_deleted_p): Remove static

(xheader_keyword_deleted_p): Remove static
qualifier.
(struct xhdr_tab.decoder): Change prototype. POSIX allows string
values to contain embedded nulls, so take an extra argument
specifying the length of the string.
(decx,decg,dummy_decoder,atime_decoder,gid_decoder)
(gname_decoder,linkpath_decoder,ctime_decoder,mtime_decoder)
(path_decoder,size_decoder,uid_decoder,uname_decoder)
(sparse_size_decoder,sparse_numblocks_decoder)
(sparse_offset_decoder,sparse_numbytes_decoder): Likewise.
(decode_record): Pass value length to the handler
(run_override_list): Pass value length to the decoder
(xheader_print_n): New function
(xheader_print): Rewritten using xheader_print_n
(xheader_finish): Do not rely om strlen to compute the length of
the collected string: it can contain embedded nulls
(xheader_string_begin,xheader_string_add,xheader_string_end): New
functions.
(sparse_map_decoder,dumpdir_coder,dumpdir_decoder): New
functions. Handle GNU.sparse.map and GNU.dumpdir variables.
(xhdr_tab): Add new variables.