build: Use -Wno-aggregate-return

It's just too noisy.
io: Resolve relative URIs
io: Use XML_PARSE_NONET

We don't want to load resources off the net.

Bug #691708.
io: Implement strict load policy

Allow any file to load from data:, and any resource to load from other
resources. Only allow file: to load other file: URIs from below the path
of the base file. Any other loads are denied.

Bug #691708.
docs: Tiny fix
build: Fix build on F19
tests: Disable failing test

This test fails in 2.37.0 already on F19.
Version 2.39.0
Post release version bump