tree-wide: Squash noncritical compiler warnings

tree-wide: Squash noncritical compiler warnings

Should fix everything from
<https://kojipkgs.fedoraproject.org//packages/ostree/2017.2/3.fc25/data/logs/x86_64/build.log>

Anything that uses autocleanups should *always* be initialized directly I think,
even if a few lines down we directly assign, since this way it's more robust
against refactoring.

And the `freopen()` warnings are right - IMO we should *always* check return
values.

Closes: #711
Approved by: jlebon