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/zcash/util.h

@@ -1,5 +1,5 @@
-#ifndef __ZCASH_UTIL_H
-#define __ZCASH_UTIL_H
+#ifndef ZC_UTIL_H_
+#define ZC_UTIL_H_
 
 #include <vector>
 #include <cstdint>
@@ -8,4 +8,4 @@ std::vector<unsigned char> convertIntToVectorLE(const uint64_t val_int);
 std::vector<bool> convertBytesVectorToVector(const std::vector<unsigned char>& bytes);
 uint64_t convertVectorToInt(const std::vector<bool>& v);
 
-#endif // __ZCASH_UTIL_H
+#endif // ZC_UTIL_H_