"tahoe cp": audit use of "mutable" flag

[warner]

David-Sarah pointed out a probable bug in source:src/allmydata/scripts/tahoe_cp.py​ , in which the self.mutable flag (set on things like TahoeFileTarget and TahoeDirectorySource) is defaulting to False when the webapi JSON data doesn't provide a value. I suspect that I lazily copy-and-pasted code from the File cases into the Directory cases, and that while older webapi data might omit mutable and should default to False for files, for directories the opposite is true, and we should default to True.

This probably won't have any practical consequence until "tahoe cp" is taught to take advantage of immutable directories (by copying the dircap instead of creating a brand new directory).