FTP and callback cleanups

I'm about to merge in a fix for the memory leak Adrian found. It was a
conflict between xfree/xcalloc use and object destruction of String
being needed to free strings.

As a side effect, I may have broken FTP, due to FTP's use of a
cbdataFree handler - which really doesn't work well with C++, as the
compiler may zero objects after delete is called, or otherwise tramp on
the memory.

Rob

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