Make default Record-Separator(--rs) value consistent.

* Default value for `--rs` is `^$|^(=====*|-----*)$`
  (blank line or four or more dashes.)
* Make a note about amlgrep's newline handling in RS that affects when
  there's consequetive empty records, and modified test case to avoid
  that. (can't think of any work-around so far, but I believe this is a
  trivial issue for most of usecases.)