mtree: replace tree structure even there are no records in db table

- reload didn't replaced old tree if no records in db table
- reported by Juha Heinanen
core: add str_append helper function
core: add check_boundaries() to build_req_buf_from_sip_req().

This checks and fixes the boundaries of a multipart body if the
FL_BODY_MULTIPART flag is set.
textops: functions to convert, append and remove multipart bodies
textops: add new multipart functions documentation
textops: Refresh README