Get rid of underscore prefixes for include guards.

This is because they may conflict with reserved names. Also took
this opportunity to make the guards in the zcash subdir a bit
more uniform by having them all conform to ZC_<...>_H_

This closes #1272

src/univalue/include/univalue.h

@@ -3,8 +3,8 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#ifndef __UNIVALUE_H__
-#define __UNIVALUE_H__
+#ifndef UNIVALUE_H__
+#define UNIVALUE_H__
 
 #include <stdint.h>
 
@@ -293,4 +293,4 @@ extern const UniValue NullUniValue;
 
 const UniValue& find_value( const UniValue& obj, const std::string& name);
 
-#endif // __UNIVALUE_H__
+#endif // UNIVALUE_H__