Expand s_all to check return values are in parentheses

It's annoying to (a) have to manually check for this when writing code, and (b) reviewing for this. Either s_style or s_clang_format could do it, preferably s_clang_format since it'll be easier to avoid false positives (e.g. a comment saying "return foo because blah").

There are a couple of places in the code that have been missed, so fix those up too.