Omit wordsplit API that tar doesn’t need

Omit wordsplit API that tar doesn’t need

* lib/wordsplit.c: Include <attribute.h> here, not in wordsplit.h.
(WRDSO_ESC_SET, WRDSO_ESC_TEST): Move here from wordsplit.h.
(WORDSPLIT_EXTRAS_extern): New macro.  Used by functions
that tar doesn’t need to be exposed.
(wordsplit_append, wordsplit_c_quoted_length, wsplt_quote_char)
(wordsplit_c_unquote_char, wordsplit_c_quote_char)
(wordsplit_c_quote_copy, wordsplit_get_words, wordsplit_perror):
Omit unless _WORDSPLIT_EXTRAS.
(WORDSPLIT_ENV_INIT): Move here from wordsplit.h, and
make it a constant rather than a macro.
(wordsplit_strerror): Arg is now pointer to const.
* lib/wordsplit.h: Do not include attribute.h, so that library
users need not worry about attribute.h.
(wordsplit_t): Declare only if _WORDSPLIT_EXTRAS.  Similarly for
functions that are not exported to tar.