Upgrade to libxml 2.9.2 and libxslt 1.1.28

third_party/libxml/src/HACKING

+NOTE: 
+  - the head of this module is package libxml-2 . There is incompatibilities
+    with the old libxml-1 headers. I posted on gnome-hackers a recipe to
+    change your code to compile with both, check it out ! Also read
+    http://xmlsoft.org/upgrade.html
+  - in the meantime the old 1.x code has been tagged with LIB_XML_1_BRANCH
+    extract this version and drop me a mail if you want me to take care of
+    the update of your module to libxml-2 <daniel@veillard.com>
+  - the 1.x branch has a separate commit policy, please check the HACKING
+    file for this branch
+
+     Rules for commits on the gnome-xml module
+     =========================================
+
+  BEFORE READING FURTHER: DO NOT COMMIT DIRECTLY !
+
+ In the exceptional case where a serious breakage in this module
+prevents other core projects from making progress, then feel free
+to patch first and send mail afterward as long as the changes are limited.
+Please keep in mind that a large part of my user base is on Windows, so
+be careful with potential portability problems there.
+
+Otherwise, send me (veillard@redhat.com) a mail and if it's a bug
+issue, register it at bugzilla.gnome.org (module libxml). I check both
+my mail and the bug database on a regular basis. If you don't get an
+answer within a week (which is highly unprobable) then commit your changes.
+This simply mean that I'm on holliday or on the road.
+
+  thanks in advance for following the rule,
+
+Daniel
+
+P.S.: Bjorn Reese, William Brack, Thomas Broyer, Igor Zlatkovic and
+      Aleksey Sanin get an exception for the send before commit rule
+      as well as John Fleck for the doc maintenance Send them mail if
+      I don't answer to request in a timely fashion
+