TaskBar+Utilities: Add logout(1) command, and call it in ShutdownDialog

logout kills the session that SystemServer --user was started with.

Userland/Utilities/logout.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/ProcessStatisticsReader.h>
+#include <signal.h>
+
+static Core::ProcessStatistics const& get_proc(Core::AllProcessesStatistics const& stats, pid_t pid)
+{
+    for (auto& proc : stats.processes) {
+        if (proc.pid == pid)
+            return proc;
+    }
+    VERIFY_NOT_REACHED();
+}
+
+int main(int, char**)
+{
+    if (pledge("stdio proc rpath", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+
+    if (unveil("/proc/all", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    if (unveil("/etc/passwd", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    unveil(nullptr, nullptr);
+
+    // logout finds the highest session up all nested sessions, and kills it.
+    auto stats = Core::ProcessStatisticsReader::get_all();
+    if (!stats.has_value()) {
+        warnln("couldn't get process statistics");
+        return 1;
+    }
+
+    pid_t sid = getsid(0);
+    while (true) {
+        pid_t parent = get_proc(stats.value(), sid).ppid;
+        pid_t parent_sid = get_proc(stats.value(), parent).sid;
+
+        if (parent_sid == 0)
+            break;
+
+        sid = parent_sid;
+    }
+
+    if (kill(-sid, SIGTERM) == -1) {
+        perror("kill(2)");
+        return 1;
+    }
+
+    return 0;
+}