refactor: Move ScheduleBatchPriority to its own file

With the previous move of AlertNotify out of the validation file, and
thus out of the kernel library, ScheduleBatchPriority is the last
remaining function used by the kernel library from util/system. Move it
to its own file, such that util/system can be moved out of the util
library in the following few commits.

Moving util/system out of the kernel library removes further networking
as well as shell related code from it.

src/util/batchpriority.cpp

@@ -0,0 +1,26 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <logging.h>
+#include <util/syserror.h>
+
+#if (defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__DragonFly__))
+#include <pthread.h>
+#include <pthread_np.h>
+#endif
+
+#ifndef WIN32
+#include <sched.h>
+#endif
+
+void ScheduleBatchPriority()
+{
+#ifdef SCHED_BATCH
+    const static sched_param param{};
+    const int rc = pthread_setschedparam(pthread_self(), SCHED_BATCH, &param);
+    if (rc != 0) {
+        LogPrintf("Failed to pthread_setschedparam: %s\n", SysErrorString(rc));
+    }
+#endif
+}