Fix warnings-as-errors for clang

Fix warnings-as-errors for clang

It seems like some versions of clang have a problem with using
pragmas to stop particular warnings being converted to errors with
-Werror. These compilers require that the warning be ignored completely
in order to suppress it.

Make the necessary changes to afsd and bozo, and update README.WARNINGS
to note the problem.

Change-Id: I66038130695d2ad27c289f29bcd8f6f2eddf1ded
Reviewed-on: http://gerrit.openafs.org/9135
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>