Xslt: disabled loading of external entities over the network

Xslt: disabled loading of external entities over the network

Loading of external entities, including ones defined with the
xml_entities directive, happens while parsing the XML response, and
therefore loading over the network can block the entire worker process
for a long time.  Loading of external DTD subset is disabled for the
very same reason since initial version of the module.

Further, loading over the network is anyway not available by default
since libxml2 2.13.0 (Jun 12 2024) and completely removed in libxml2
2.15.0 (Sep 15 2025).

As such, the XML_PARSE_NONET parsing option (available since libxml2
2.6.2 from 2003) is now used to prevent loading of external entities
over the network.

Signed-off-by: Vadim Zhestikov <v.zhestikov@f5.com>
Origin: https://freenginx.org/hg/nginx/rev/081c50f47347