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]
diff --git a/target/riscv/pmp.h b/target/riscv/pmp.h
index e3953c8..6679095 100644
--- a/target/riscv/pmp.h
+++ b/target/riscv/pmp.h
@@ -19,8 +19,8 @@
  * this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef _RISCV_PMP_H_
-#define _RISCV_PMP_H_
+#ifndef RISCV_PMP_H
+#define RISCV_PMP_H
 
 typedef enum {
     PMP_READ  = 1 << 0,