KPath: Added LAZY_ALLOC flag.

* This allows KPath to not allocate a buffer when initialized
without path.
* Added test cases for this.
* Added test for LockBuffer().
* Enhanced tests to allow building them in debug mode.
* Moved calling vfs_normalize_path() into own private method.
* Improved error codes; B_NO_MEMORY is now only returned if the
allocation actually failed.
* If used with LAZY_ALLOC, Path() and LockBuffer() are now allowed
to return a NULL path.

KPath: Fixed Normalize() return code, changed Leaf().

* Normalize() now returns the error code that vfs_normalize_path()
returns.
* Leaf() now returns "" instead of "/" for the root. It's not used
outside of KPath.
* Adapted RemoveLeaf() to deal with this correctly.
* "KPath = string" no longer changes the buffer size.
* Added missing operator tests for =, ==, and !=.

Added unit tests for KPath.

* There are a few oddities, and at least one bug.