Include blank lines in messages edited with internal editor

Needed a version of strListSplit() that doesn't treat consecutive delimiters (e.g. "\r\n\r\n") as a single delimiter: so introducing strListDivide(). Like strListSplit(), strListDivide() modifies the input string (replacing first occurrences of delimiter with NUL).

Since we don't want "\r\n" to be counted as 2 lines, we need to just split/divide on '\n' and then truncate any line endings (i.e. '\r') chars off the ends of the split strings.

Reported by phigan in #synchronet