Squash warnings for warn_unused_result.
Clean up mistakes from d63c53e.

Add missing braces, avoid unnecessary variables.

Noticed by:	fgs
Squash further warn_unused_result warnings.

Not being able to fchown() is normal when running varnishd
unprivileged, so don't report failure unless running as root.
Relax the vmod abi check if we are not in master

For master we retain the more strict check in case we forget to bump
the API version as discussed on irc.  Implementation idea from scoof.

Fixes #1538