[xml] libxml string handling

[Stefan Kost <kost imn htwk-leipzig de>]

a while ago, there ws a discussion about string handling in libxml (too many strdups).
what about this approach :

>>>>
from : http://www.redland.opensource.ac.uk/docs/api/objects.html

It is often convienent to get a string representation of an object for
further processing, debugging or serialisation. There are two
conventions for methods that provide this functionality - the first is
when a pointer is returned to a shared copy of the string and in that
case the method ends _as_string. The second is when a pointer is
returned to a newly allocated string in which case the method ends
_to_string.
<<<<

so we basically rename all the currently affected methods to end with _as_string
and just return the pointer plu we provided functions _ending in _to_string
which call the _to_string and strdup it. We further provided #defines to redirect
calls of the original name to the _to_sting function plus issuing
a compiler warning about deprecation via #warning.

I am not totally sure which way to go for functions which generate the
resulting string. One can aleways use a static buffer therefore,
but that can easilly lead to stange effect (one can only use it as long
as it not calls the function again).

Stefan
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