Explicitly load external entities (after CVE-2014-0191)

Use the more modern API function for XML loading and enable the
flags which load the external entities, so that meinproc4 can work
again after the security changes implemented for CVE-2014-0191.
Network loading is disabled too now.

REVIEW: 118270
BUG: 335001
FIXED-IN: 4.13.2