Clean up ill-advised or unusual header guards

Leading underscores are ill-advised because such identifiers are
reserved.  Trailing underscores are merely ugly.  Strip both.

Our header guards commonly end in _H.  Normalize the exceptions.

Done with scripts/clean-header-guards.pl.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190315145123.28030-7-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Changes to slirp/ dropped, as we're about to spin it off]
38 files changed