(struct xhdr_tab.keyword): Now pointer to const.

(struct xhdr_tab.keyword): Now pointer to const.
(struct xhdr_tab.coder, struct xhdr_tab.decoder, locate_handler,
decode_record, xheader_store, xheader_print, code_string, code_time,
code_num, dummy_coder, dummy_decoder, atime_coder, atime_decoder,
gid_coder, gid_decoder, gname_coder, gname_decoder, linkpath_coder,
linkpath_decoder, ctime_coder, ctime_decoder, mtime_coder,
mtime_decoder, path_coder, path_decoder, size_coder, size_decoder,
uid_coder, uid_decoder, uname_coder, uname_decoder):
Use pointers to const when possible.
(xhdr_tab): Now const.

(struct xhdr_tab.decoder, dummy_decoder,
atime_decoder, gid_decoder, gname_decoder, linkpath_decoder,
ctime_decoder, mtime_decoder, path_decoder, size_decoder,
uid_decoder, uname_decoder): Remove unused keyword arg.
All uses changed.

(xhdr_tab, format_uintmax): Now static.

(dummy_coder, dummy_decoder, atime_coder,
atime_decoder, gid_coder, gid_decoder, gname_coder, gname_decoder,
linkpath_coder, linkpath_decoder, mtime_coder, mtime_decoder,
ctime_coder, ctime_decoder, path_coder, path_decoder, size_coder,
size_decoder, uid_coder, uid_decoder, uname_coder, uname_decoder):
Remove forward decls; no longer needed.
(xhdr_tab): Move to end, so that the forward decls aren't needed.
Add a forward declaration.

(decode_record): Return bool, not int.

(decode_record): Fix misspelling in diagnostic
"extended headed" -> "extended header".

(decode_record, xheader_decode):
Do not use decls like "char *p, *q;".