The file input callbacks try to read from stdin if "-" is passed as URL.
This should never be done when loading indirect resources like external
entities or XIncludes. Unfortunately, the stdin substitution happens
deep inside the IO code, so we simply replace "-" with "./-" in specific
locations.

This issue also affects other users of the library like libxslt.
Ideally, stdin should only be substituted on explicit request. But more
intrusive changes could break existing code.

Closes #90 and #102.