Allow uploading files even when permissions cannot be set.

[alip]

By default, the "file" function uses sftp to upload the target file to the server. During the upload, the Net::SFTP::Foreign runs chmod unconditionally, failing if the target file is not owned by the ssh user, unless that user is a root. This commit changes that behavior by passing an optional "best_effort" flag that instructs sftp to continue even if setting permissions has failed.

As a result, an unprivileged user will be able to use Rex scenarios that call the "file" function without getting permission denied errors.

This PR is an attempt to fix # by .

Checklist

• [ ] based on top of latest source code
• [ ] changelog entry included
• [ ] tests pass on Travis CI
• [ ] git history is clean
• [ ] git commit messages are well-written