Rsync broken on MacOS

[sbliven]

As mentioned in the comments of #43, MacOS ships with rsync 3.2.7 but doesn't support the --msg2stderr parameter. It should use --stderr=error as well, similar to RHEL9 (see #53).

In general, I think we need better rsync capability detection and test this on lots of versions. @kavir1698 looked into libraries for this and didn't find anything current, but maybe we can update one.

[kavir1698]

An option is to use a go package for file transfers. Choices include sftp and go-scp. SFTP is the more featureful option.

Both alternatives would need more coding to replicate rsync's features, specifically the --files-from and --progress flags.

[kavir1698]

Another solution is to write a Linux version of syncDataToFileserver_windows.go to avoid using rsync.