Adding ioutil.TempDir support to make unique temp directories

[nicknezis]

No description provided.

[joelanford]

There's actually some complications with this change. The creation of the directory is happening in the scratch.Setup() function for non-ramdisk stuff and in the ramdisk-specific attach or mount functions when ramdisk is enabled. So that needs to be cleaned up.

I also noticed that I'm not always checking to make sure that the directory does not exist before creating it. That part is important. It tells me its safe to delete it when I'm done, because there shouldn't be any other user files in there.

I'm thinking we could just pass the scratchSpaceDir option directly into scratch.New() without modification and then cleanup the scratch.Setup function to handle the creation of that directory in just one place. If that option is an empty string, we just use ioutil.TempDir("/tmp", "goscan") and if not, we check that whatever was passed in doesn't exist and then create it.

Then the only other thing to worry about is that the filescanner.Opts needs to know what that directory is, so we would need to make a new function on Scratch to return the directory created after Setup is called.

[nicknezis]

Replacing with this: #3

Basically squashed the commits and removed a merge done by mistake.