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/prf.h

@@ -3,8 +3,8 @@ Zcash uses SHA256Compress as a PRF for various components
 within the zkSNARK circuit.
 */
 
-#ifndef _PRF_H_
-#define _PRF_H_
+#ifndef ZC_PRF_H_
+#define ZC_PRF_H_
 
 #include "uint256.h"
 #include "uint252.h"
@@ -15,4 +15,4 @@ uint256 PRF_nf(const uint252& a_sk, const uint256& rho);
 uint256 PRF_pk(const uint252& a_sk, size_t i0, const uint256& h_sig);
 uint256 PRF_rho(const uint252& phi, size_t i0, const uint256& h_sig);
 
-#endif // _PRF_H_
+#endif // ZC_PRF_H_