[1165] Option value expansion implementation

In libsvn_subr/config.c's make_string_from_option() function, to
implement expansion I need to know which section that the option which
we're making a string for is coming from.

[section]
X=foo
Y=%(X)s

When expanding the value of X for option Y, I need know which section
option Y comes from so that I can look their for option X. I see two
ways of implementing this:

1) Added a cfg_section_t * to the cfg_option_t structure (set in the
svn_config_set() function), so that an option knows which section it
comes from. This would require moving the block in svn_config_set()
which creates a section if one doesn't already exist to before the
instantiation of the option.

2) Leverage the last parameter of find_option() (a cfg_section_t **),
and expand the API of make_string_from_option() to take a
cfg_section_t * parameter. find_option() seems to always be called
before make_string_from_option().

Both are fairly easy to implement, but I'm not sure which would be
most appropriate and am hoping for some feedback.

-- 
Daniel Rall <dlr@finemaltcoding.com>
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