LibCore+LibIPC: Move SystemServerTakeover.{h,cpp} to LibCore

This functionality is required by Core::LocalServer and LibIPC depends
on LibCore.

take_over_accepted_socket_from_system_server has also been renamed to
take_over_socket_from_system_server as the socket need not be accepted
before taking it over. :^)

Userland/Libraries/LibCore/SystemServerTakeover.cpp

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2022, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "SystemServerTakeover.h"
+#include <LibCore/System.h>
+
+namespace Core {
+
+HashMap<String, int> s_overtaken_sockets {};
+bool s_overtaken_sockets_parsed { false };
+
+static void parse_sockets_from_system_server()
+{
+    VERIFY(!s_overtaken_sockets_parsed);
+
+    constexpr auto socket_takeover = "SOCKET_TAKEOVER";
+    const char* sockets = getenv(socket_takeover);
+    if (!sockets) {
+        s_overtaken_sockets_parsed = true;
+        return;
+    }
+
+    for (auto& socket : StringView(sockets).split_view(' ')) {
+        auto params = socket.split_view(':');
+        s_overtaken_sockets.set(params[0].to_string(), strtol(params[1].to_string().characters(), nullptr, 10));
+    }
+
+    s_overtaken_sockets_parsed = true;
+    // We wouldn't want our children to think we're passing
+    // them a socket either, so unset the env variable.
+    unsetenv(socket_takeover);
+}
+
+ErrorOr<NonnullOwnPtr<Core::Stream::LocalSocket>> take_over_socket_from_system_server(String const& socket_path)
+{
+    if (!s_overtaken_sockets_parsed)
+        parse_sockets_from_system_server();
+
+    int fd;
+    if (socket_path.is_null()) {
+        // We want the first (and only) socket.
+        VERIFY(s_overtaken_sockets.size() == 1);
+        fd = s_overtaken_sockets.begin()->value;
+    } else {
+        auto it = s_overtaken_sockets.find(socket_path);
+        if (it == s_overtaken_sockets.end())
+            return Error::from_string_literal("Non-existent socket requested"sv);
+        fd = it->value;
+    }
+
+    // Sanity check: it has to be a socket.
+    auto stat = TRY(Core::System::fstat(fd));
+
+    if (!S_ISSOCK(stat.st_mode))
+        return Error::from_string_literal("The fd we got from SystemServer is not a socket"sv);
+
+    auto socket = TRY(Core::Stream::LocalSocket::adopt_fd(fd));
+    // It had to be !CLOEXEC for obvious reasons, but we
+    // don't need it to be !CLOEXEC anymore, so set the
+    // CLOEXEC flag now.
+    TRY(socket->set_close_on_exec(true));
+
+    return socket;
+}
+
+}