lockedpool: avoid sensitive data in core files (FreeBSD)

This is a followup to
23991ee53 / https://github.com/bitcoin/bitcoin/pull/15600
to also use madvise(2) on FreeBSD to avoid sensitive data allocated
with secure_allocator ending up in core files in addition to preventing
it from going to the swap.

src/support/lockedpool.cpp

@@ -253,8 +253,10 @@ void *PosixLockedPageAllocator::AllocateLocked(size_t len, bool *lockingSuccess)
     }
     if (addr) {
         *lockingSuccess = mlock(addr, len) == 0;
-#ifdef MADV_DONTDUMP
+#if defined(MADV_DONTDUMP) // Linux
         madvise(addr, len, MADV_DONTDUMP);
+#elif defined(MADV_NOCORE) // FreeBSD
+        madvise(addr, len, MADV_NOCORE);
 #endif
     }
     return addr;