Allow us to use the overwrite option for a symlink. This will still fail if the destination already exists as another file, but if it's a symlink it will overwrite it.
Check for symlink first, and overwrite if we have set that option
Let's make overwrite the default for symlinks so we can run this against an existing tree (it'll fail if there are symlinks otherwise)
Er, change overwrite to True for symlinks, not empty directories
[mthaddon] Check for symlink first and overwrite unless explicitly asked not to.