Re: Another type issue.

We should use svn_string_t whenever:

        * We are passing a mutable string, or
        * We need to represent binary data (including 0 bytes)

We should use char * whenever the above aren't true, i.e. when we're
passing an immutable string which can only contain text data.

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That's my position.  Some rationale:
We could use svn_string_t everywhere.  It would be kind of a pain in
the butt (because we'd have to convert every time we wanted to use a
system call or apr string-handling routine, and because we'd have to
worry about memory allocation even when we were just passing a
literal), but it would work.  But we've rejected that approach.
Frankly, I think it's a tiny bit unclean to pass an svn_string_t * for
an immutable string when we are representing binary data.  But blame C
for using a half-assed string type for that gaffe.  A "const"
qualifier will make it clear that the called function isn't allowed to
modify the string.