osfs: add file.Sync() support

[georg]

Adds a call to file.Sync() to util.WriteFile() (right before closing the file, so it should be functionally equivalent) to ensure test coverage.

This is a continuation of PR #126 and implements the suggested changes.

Fixes: #86
Closes: #126

[pjbgf]

@georg thanks for following up on this.

[georg]

@pjbgf Thanks for the review. I've made the changes. Only BoundOS now reports the SyncCapability.

For ChrootOS it's a bit trickier.
I think it would report the SyncCapability correctly if the underlying FS reports it but its file implementation does not have a Sync method. I could change the chroot.newFile() implementation to return a different file type that implements Syncer if the underlying FS has the SyncCapability. What do you think?

Then there's also polyfill.Polyfill which ChrootOS uses when you initialise a new FS with os.New(dir). Polyfill has no way of finding out the underlying OS capabilities and just returns the DefaultCapabilities - see billy.Capabilities(). The comment of billy.Capabilities() is not correct at the moment because it is not returning all capabilities but just the default capabilities. We could return all capabilities here which would make the ChrootOS case work when polyfill is used, but I haven't looked into other side effects of changing billy.Capabilities().

[pjbgf]

@georg thanks for looking into this. Don't worry about the ChrootOS implementation for the time being. As part of the V6 there may be some restructuring around osfs. Historically we only added a second implementation of osfs to not break backwards compatibility, so one of them may cease to exist before the first version for v6 is cut. I will create an issue to review this.