gnu: libxml2: Add fixes for CVE-2015-{1819,7941,7942,8035} and other bugs.

* gnu/packages/patches/libxml2-CVE-2015-1819.patch,
  gnu/packages/patches/libxml2-CVE-2015-7941-pt1.patch,
  gnu/packages/patches/libxml2-CVE-2015-7941-pt2.patch,
  gnu/packages/patches/libxml2-CVE-2015-7942-pt1.patch,
  gnu/packages/patches/libxml2-CVE-2015-7942-pt2.patch,
  gnu/packages/patches/libxml2-CVE-2015-8035.patch,
  gnu/packages/patches/libxml2-bug-737840.patch,
  gnu/packages/patches/libxml2-bug-738805.patch,
  gnu/packages/patches/libxml2-bug-746048.patch,
  gnu/packages/patches/libxml2-bug-747437.patch,
  gnu/packages/patches/libxml2-bug-751603.patch,
  gnu/packages/patches/libxml2-bug-751631.patch,
  gnu/packages/patches/libxml2-bug-754946.patch,
  gnu/packages/patches/libxml2-bug-754947.patch,
  gnu/packages/patches/libxml2-bug-755857.patch,
  gnu/packages/patches/libxml2-fix-catalog-corruption.patch,
  gnu/packages/patches/libxml2-id-attrs-in-xmlSetTreeDoc.patch,
  gnu/packages/patches/libxml2-node-sort-order-pt1.patch,
  gnu/packages/patches/libxml2-node-sort-order-pt2.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/xml.scm (libxml2)[source]: Add patches.