VFS needs some small updates... pass in the bound fops as well as the
context fops to the member callbacks. ZIP_FOPS only cared about doing
operations on the platform / context vfs to walk the ZIP file, but other
uses are valid where we are doing operation inside the bound VFS itself.
Also, stash a cx pointer into file ops struct for convenience.
Although many of the examples must be run from the example directory as
cwd, everyone getting started probably wants to try the examples, cmake
knows how to do it, so let's enable it by default.
