[PATCH 0 of 2] XSLT filter improvements

[Maxim Dounin]

Hello!

The following patch series addresses some shortcomings of the XSLT filter
module (ngx_http_xslt_module), notably:

- Attempts to load external entities over network are now rejected.
  These anyway won't work with libxml2 2.13.0 or later, but with older
  versions of the library attempts to load external entities over network
  might block the entire worker process for a long time, and therefore
  shouldn't be used.

- Loading of external entities from the internal DTD subset (that is,
  the XML document itself) via system identifiers are now forbidden by
  default, making it slightly safer to use the module with untrusted XML
  documents.  Loading via public identifiers (with appropriate system
  XML catalogs) and from the external DTD subset (provided with the
  "xml_entities" directive) still works as usual.  If loading of external
  entities from the internal DTD subset via system identifiers is
  intentional, the "xml_external_entities" directive can be used to
  re-enable loading of such entities.

Review and testing appreciated.

-- 
Maxim Dounin