Make os.linesep default line_terminator in to_clipboard
closes #11720
- Make os.linesep default line_terminator in to_clipboard
- Add line_terminator kw to Series.to_csv
- Refactor to_clipboard

Observed/fixed problems in to_clipboard:
- try..except: pass
- excel=None, though it defaults to True
- several code paths with return
- sep argument could be passed via kwargs
- "other keywords are passed to to_csv" only if excel=True
- line_terminator kw isn't supported by Series to_csv

Is it ok to to change Series.to_csv api?
Also we need tests, right? But what does `# pragma: no cover` mean?
