Implement _get_lastbday
Orthogonal to other PRs.

Implement `_get_lastbday` mirroring `_get_firstbday`.

Add a dummy argument to get_firstbday so that the signatures match.  Following this and #18218, we'll be able to define e.g.

```
class BQuarterBegin(...):
    _get_bday = _get_firstbday

class BMonthEnd(...):
    _get_bday = _get_lastbday
```

etc.  From there we can get rid of _bunch_ of duplicated logic.

