Fix file and line report for XSD SAX and reader streaming validation

Things now work correctly at the xmllint level:
thinkpad:~/XML -> xmllint --sax --noout --schema test_schema.xsd
test_xml.xml
test_xml.xml:72721: Schemas validity error : Element 'level1': Missing
child element(s). Expected is ( level2 ).
test_xml.xml fails to validate
thinkpad:~/XML -> xmllint --stream --schema test_schema.xsd test_xml.xml
test_xml.xml:72721: Schemas validity error : Element 'level1': Missing
child element(s). Expected is ( level2 ).
test_xml.xml fails to validate
thinkpad:~/XML ->

* error.c: fix a corner case of not reporting lines when we should
* include/libxml/xmlschemas.h doc/symbols.xml: had to add new entry
  points to set the filename on a validation context and a locator
  callback used to fetch the line and file from the context
* xmlschemas.c: add the new entry points xmlSchemaValidateSetFilename()
  and xmlSchemaValidateSetLocator(), plus make sure the error reporting
  routine gets the information if available. Add a locator for SAX.
* xmlreader.c: add and plug a locator for readers.