Sync only *.py, *.pyx, *.so in saltutil.sync_*

**Note: if we decide to get this into 2014.1.0, please coordinate with me so I can add ``.. versionchanged::`` directives to this pull req before you merge. This pull adds new parameters to a CLI-facing function, and I want to properly document this.**

This avoids caching anything but the file types that are needed by
saltutil.sync_*. Resolves #10160.