v2: msc_xml.c prints libxml2 errors to stderr and discards the parse reason #3680

What

apache2/msc_xml.c on v2/master has neither of the two libxml2 error-handling changes that just went into v3 (#3677 and #3679):

1. libxml2 parser errors go to stderr. Both push parser contexts (msc_xml.c L250 and L257) are created without XML_PARSE_NOERROR | XML_PARSE_NOWARNING, and nothing installs an error handler. libxml2 therefore prints body.xml:1: parser error : ... plus the context lines through its default handler, which under Apache ends up in the error log without the usual [date] [module:level] [pid] prefix. This is the v2 version of REQBODY_ERROR "Extra content at the end of the document" after 3.0.15 update #3563, and it has always been there rather than being a regression.
2. The reason is discarded. On failure the engine reports the fixed strings XML: Failed to parse document. / XML: Failed to parse document for ARGS. (L276, L288, L327, L339), so REQBODY_ERROR_MSG never says whether it was a tag mismatch, extra content after the root element, an empty document, and so on.

The original design kept the reason: xml_receive_sax_error() at L204 formats lastError.message (line N offset M) into the debug log, but the block that wires it up (L236-L247) is commented out, so the function is dead code.

Proposal

Same two changes as v3, in one or two commits:

• After each xmlCreatePushParserCtxt(), call xmlCtxtUseOptions(ctx, XML_PARSE_NOWARNING | XML_PARSE_NOERROR). This is the call that works on every libxml2 version; OR-ing ctx->options directly is a no-op before libxml2 2.13 (see fix: silence libxml2 parser errors on every libxml2 version #3677 for the measurements).
• At the four failure sites, build *error_msg from xmlCtxtGetLastError(ctx) (message, line, int2), trimming the trailing newline, with the fixed string as the fallback when there is no message. In xml_complete() read it before xmlFreeParserCtxt(). Reuse or delete the dead xml_receive_sax_error() rather than keeping a third variant.

xmlCtxtGetLastError() keeps working under XML_PARSE_NOERROR, is available since libxml2 2.6, and is not deprecated in 2.15.

Test

tests/regression/ has no XML parser error case today. Add one under tests/regression/target/ that posts a body with a tag mismatch, sets ctl:requestBodyProcessor=XML, and matches REQBODY_ERROR_MSG with @contains Opening and ending tag mismatch. The Perl runner auto-discovers new .t files.

References

v3 counterparts: #3677 (silence stderr), #3679 (surface the message). Background: #3563, #3678.