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/ipc/interfaces.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <common/system.h>
 #include <interfaces/init.h>
 #include <interfaces/ipc.h>
 #include <ipc/capnp/protocol.h>
@@ -10,7 +11,6 @@
 #include <logging.h>
 #include <tinyformat.h>
 #include <util/fs.h>
-#include <util/system.h>
 
 #include <cstdio>
 #include <cstdlib>