Consolidating private/svn_subr_private.h

Hi there,

While looking for the appropriate place to declare a few
svn-private constants, I realized that libsvn_subr is the
only lib that comes with multiple private headers.

So, I prepared a patch that moves all their content into
private/svn_subr_private.h with two exceptions:

svn_sqlite.h and svn_magic.h

These are wrappers around 3rd partly libs so I left them
alone.

Right now seems to perfect time for that kind of cleanup
as the old release is reasonably stable and the new
stabilization branch has not been created yet. If there are
no strong objections to the change, I would commit the
patch this weekend.

-- Stefan^2.

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