(wordexp): Perform word-splitting instead of field-splitting here.

(wordexp): Perform word-splitting instead of field-splitting here.
(wordexp): If out of memory mid-word, free the word (but still
leave pwordexp alone for caller to see).
(parse_param): Allow for zero-length fields (smarter checking of
memory allocation failure).
(w_addword): Convert NULL words to .
(wordexp): Convert left-over IFS characters to blanks (like bash).