Adjust all struct dirent creations (again), this time to use offsetof().

The dirent struct is not packed, so offsetof(dirent, d_name) != sizeof(dirent).
Thus in order not to waste the alignment bytes (which are significant,
on x86_64 at least, sizeof(dirent)==32, but offsetof(...)=26.)

This is also the most portable way to handle things, and should
work just fine in cross-platform code that has a non-zero-sized d_name.

BCopyEngine: Add base class BEntryOperationEngineBase

It contains a helper class Entry, which allows to generalize the
BCopyEngine::CopyEntry() parameters.

Add private storage kit class BCopyEngine

It provides the functionality to copy file system entries (also
recursively). The code originates from the copyattr sources. Some
copyattr specific functionality has been removed and the code has been
adjusted for library use (i.e. no exit()s or fprintf()s). An optional
controller object can be set to customize the behavior.

BCopyEngine: Add base class BEntryOperationEngineBase

It contains a helper class Entry, which allows to generalize the
BCopyEngine::CopyEntry() parameters.

Add private storage kit class BCopyEngine

It provides the functionality to copy file system entries (also
recursively). The code originates from the copyattr sources. Some
copyattr specific functionality has been removed and the code has been
adjusted for library use (i.e. no exit()s or fprintf()s). An optional
controller object can be set to customize the behavior.