parser: add internal flag FL_BODY_MULTIPART

This will be set if the body has been converted to multipart/mixed
or a new part has been appended to it. This will trigger the core
to check and fix the boundaries of the body.
rr: use internal msg flags to mark a request with record route added
core: new internal msg flag added - FL_RR_ADDED

- marks a sip request when record route is added
textopsx: msg_apply_changes() returns error if record route was added
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