xml.c: Update deprecated libxml2 API usage.

xml.c: Update deprecated libxml2 API usage.

Two functions are deprecated as of libxml2 2.12:

  * xmlSubstituteEntitiesDefault
  * xmlParseMemory

So we update those with supported API.

Additionally, `res_calendar_caldav` has been updated to use libxml2's
xmlreader API instead of the SAX2 API which has always felt a little
hacky (see deleted comment block in `res_calendar_caldav.c`).

The xmlreader API has been around since libxml2 2.5.0 which was
released in 2003.

Fixes #725

(cherry picked from commit f9a359c5c58b9e928f0038b952c8277f164f9a32)