Document our use of attribute((cleanup)), and clean up code.

[stefwalter]

We allow ourselves the use of GCC extensions, such as attribute((cleanup)) to simplify control flow due to temporary allocations.

This should be documented, including the replacement of the common "if (error) goto out;" idiom with "if (error) return;"

Then all code should be cleaned up to use this pattern. This might need extensions to libgsystem, or a more local approach.

[stefwalter]

I don't think rewriting everything to be consistent is going to happen. The use of the cleanup attribute is primarily a coding style in the backend.