Enhance scp.py for scp folder

[liliangnike]

Enhance scp.py to make the bahaviors to be identical to native scp command behaviors:

1. When meets non-regular files, scp.py should prompt.
2. After prompt, scp.py should continue to copy other remaining non-regular files.

[remram44]

Please stop sending PRs over and over. You can update an existing one.

[liliangnike]

Ok. Thanks. But really sorry about that.

[liliangnike]

Any comment on code change? Thanks!

[remram44]

I'm worried that this will ignore too many errors, not just the "non-regular file" errors. At the minimum this should be configurable, and it would be great to have tests.

[liliangnike]

When copying directories, native scp command ignores all the non-regular file errors. But there is error log on this. The key point is that native scp command can continue to copy all the regular files in the remaining files under this folder.

For recursive case, what are the other error code values (normally, they are C, T, D and E) when non-regular files are met?