Support for rename on servers not supporting posix-rename@openssh.com

[wrvdklooster]

At this moment sftppretty supports rename via posix-rename. This works on most servers but for example the SFTPServer in Paramiko we use for testing does not support it.

Might be an idea to have a fallback to the standard SFTP rename in case the server does not support posix-rename@openssh.com either automatically or by passing an optional parameter to the current rename.

Should not be an issue on most servers so no high priority. For now will extend the SFTPServer in Paramiko to support the posix-rename also probably.

[byteskeptical]

Technically this can also be achieved through the execute function though I can see the value of a direct fallback. Updated the #76 PR to include a posix boolean that can be used to switch between the two types of rename functions offered by paramiko.