refactor: Move system from util to common library

Since the kernel library no longer depends on the system file, move it
to the common library instead in accordance to the diagram in
doc/design/libraries.md.

src/common/system.h

@@ -0,0 +1,38 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_COMMON_SYSTEM_H
+#define BITCOIN_COMMON_SYSTEM_H
+
+#if defined(HAVE_CONFIG_H)
+#include <config/bitcoin-config.h>
+#endif
+
+#include <compat/assumptions.h>
+#include <compat/compat.h>
+
+#include <set>
+#include <stdint.h>
+#include <string>
+
+// Application startup time (used for uptime calculation)
+int64_t GetStartupTime();
+
+void SetupEnvironment();
+bool SetupNetworking();
+#ifndef WIN32
+std::string ShellEscape(const std::string& arg);
+#endif
+#if HAVE_SYSTEM
+void runCommand(const std::string& strCommand);
+#endif
+
+/**
+ * Return the number of cores available on the current system.
+ * @note This does count virtual cores, such as those provided by HyperThreading.
+ */
+int GetNumCores();
+
+#endif // BITCOIN_COMMON_SYSTEM_H