common: New function tokenize_to_strlist.
* common/strlist.c (append_to_strlist_try): Factor code out to ... (do_append_to_strlist): new. (tokenize_to_strlist): New. * common/t-strlist.c (test_tokenize_to_strlist): New. (cherry picked from commit d2dca58338a4936b293c3ec6be4572d0e74b6a0d)