Port SVN to Neon 0.24 (including SSL support)

Neon 0.24.0 no longer supports the ne_xml_elm structure, Subversion
cannot be compiled with it. The URL is the elevant change in ne_xml.h,
and the log message accompanying the change is below. Subversion's DAV
layer has to use the latest Neon functionality.
--
Redesign of the XML interface.  Major changes:
* don't require up-front registration of list of elements to handle
* have the start-element callback either accept, decline, abort,
or return a state integer.
* remove 'struct ne_xml_elm'; callbacks are passed {nspace, name}
strings along with a state integer.
* dropped "collect", "strip-leading-whitespace" modes
* push responsibility for accumulating cdata onto caller; drop 'cdata'
argument from end-element callback.
* don't abort if no handler accepts a particular element, just ignore
that branch of the tree.

http://cvs.webdav.org/viewcvs/neon/src/ne_xml.h.diff?r1=1.42&r2=1.43