Support directory copying

Note: symbolic links are not yet supported, so this implementation
may fail when symbolic links are present. For example, a symbolic
link could create an apparently infinitely deep directory structure
and the recursive directory copy algorithm would fail.
Support copying a file to itself
Support copying of symbolic links

Symbolic links must point within the directory being copied otherwise
Copy returns an error.

Add a format string variant of gerror.New.
Document diagnostic policies
Isolate syscall interface for testing.
Correctly use Syscall interface in rootfs.Generate
Fix merge error.
Reimplement Syscall stub to remember all calls.