Merge branch 'mh/strbuf-split'

Cleanups and documentation for strbuf_split.

* mh/strbuf-split:
  strbuf_split*(): document functions
  strbuf_split*(): rename "delim" parameter to "terminator"
  strbuf_split_buf(): simplify iteration
  strbuf_split_buf(): use ALLOC_GROW()