Split memclear into two versions

secp256k1_memclear has the side effect of undefining bytes for
valgrind checks. In some cases, we may want to zero bytes
but allow subsequent reads. So we split memclear into
memclear_explicit, which makes no guarantees about the content
of the buffer on return, and memzero_explicit, which guarantees
zero value on return.

Change the memset in partial_sign to use memzero_explicit.

src/modules/musig/session_impl.h

@@ -385,10 +385,10 @@ static void secp256k1_nonce_function_musig(secp256k1_scalar *k, const unsigned c
         secp256k1_scalar_set_b32(&k[i], buf, NULL);
 
         /* Attempt to erase secret data */
-        secp256k1_memclear(buf, sizeof(buf));
+        secp256k1_memclear_explicit(buf, sizeof(buf));
         secp256k1_sha256_clear(&sha_tmp);
     }
-    secp256k1_memclear(rand, sizeof(rand));
+    secp256k1_memclear_explicit(rand, sizeof(rand));
     secp256k1_sha256_clear(&sha);
 }
 
@@ -518,7 +518,7 @@ int secp256k1_musig_nonce_gen_counter(const secp256k1_context* ctx, secp256k1_mu
     if (!secp256k1_musig_nonce_gen_internal(ctx, secnonce, pubnonce, buf, seckey, &pubkey, msg32, keyagg_cache, extra_input32)) {
         return 0;
     }
-    secp256k1_memclear(seckey, sizeof(seckey));
+    secp256k1_memclear_explicit(seckey, sizeof(seckey));
     return 1;
 }
 
@@ -679,7 +679,7 @@ int secp256k1_musig_partial_sign(const secp256k1_context* ctx, secp256k1_musig_p
     ret = secp256k1_musig_secnonce_load(ctx, k, &pk, secnonce);
     /* Set nonce to zero to avoid nonce reuse. This will cause subsequent calls
      * of this function to fail */
-    memset(secnonce, 0, sizeof(*secnonce));
+    secp256k1_memzero_explicit(secnonce, sizeof(*secnonce));
     if (!ret) {
         secp256k1_musig_partial_sign_clear(&sk, k);
         return 0;