Fix xmlelement() to initialize libxml correctly before using it, and to avoid

assuming that evaluation of its input expressions won't change the state of
libxml.  This requires refactoring xml_init() to not call xmlInitParser(),
since now not all of its callers want that.  I also tweaked things to avoid
repeated execution of one-time-only tests inside xml_init(), though this is
mostly for clarity rather than in hopes of saving any noticeable amount of
runtime.  Per report from Sheikh Amjad and subsequent discussion.
In passing, fix a couple of inadequately schema-qualified queries.