ipc, refactor: Add ProcessId type alias and use it

Use ProcessId type instead of int to represent process ids to be
compatible with an upcoming version of libmultiprocess which adds
windows support.

src/ipc/interfaces.cpp

@@ -62,7 +62,7 @@ public:
     }
     std::unique_ptr<interfaces::Init> spawnProcess(const char* new_exe_name) override
     {
-        int pid;
+        mp::ProcessId pid;
         int fd = m_process->spawn(new_exe_name, m_process_argv0, pid);
         LogDebug(::BCLog::IPC, "Process %s pid %i launched\n", new_exe_name, pid);
         auto init = m_protocol->connect(fd);