Now that hanging brace offenders have been fixed, we can automate
the check, and document our style.  Done as a separate commit from
code changes, to make it easier to just backport code changes, if
that is ever needed.

* cfg.mk (sc_curly_braces_style): Catch hanging braces.
* docs/hacking.html.in: Document it.
* HACKING: Regenerate.
Signed-off-by: NEric Blake <eblake@redhat.com>