CODING: one-line if statements should not have braces

CODING: one-line if statements should not have braces

Update the style guide with a declaration of the prevailing and
preferred brace style for one-line if statements and loops. Provide an
example and counter-example.

Change-Id: Iafeea977203b76c0e67385779fb4ed57f3c6699a
Reviewed-on: https://gerrit.openafs.org/12370
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>