LogManager: implement system module, client api, logging api (#1617)

Some notes:

* Unless `atmosphere!enable_log_manager` is true, Nintendo's log manager will be used instead.
  * This prevents paying memory costs for LM when not enabling logging.
  * To facilitate this, Atmosphere's log manager has a different program id from Nintendo's.
  * `atmosphere!enable_htc` implies `atmosphere!enable_log_manager`.
* LogManager logs to tma, and the SD card (if `lm!enable_sd_card_logging` is true, which it is by default).
* Binary logs are saved to `lm!sd_card_log_output_directory`, which is `atmosphere/binlogs` by default.

libraries/libstratosphere/include/stratosphere/diag/diag_log_observer.hpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2018-2020 Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+#include <vapours.hpp>
+#include <stratosphere/diag/diag_log_types.hpp>
+
+namespace ams::diag {
+
+    using LogObserver = void (*)(const LogMetaData &meta, const LogBody &body, void *arg);
+
+    struct LogObserverHolder {
+        LogObserver log_observer;
+        LogObserverHolder *next;
+        bool is_registered;
+        void *arg;
+    };
+
+    constexpr inline void InitializeLogObserverHolder(LogObserverHolder *holder, LogObserver observer, void *arg) {
+        holder->log_observer  = observer;
+        holder->next          = nullptr;
+        holder->is_registered = false;
+        holder->arg           = arg;
+    }
+
+    void RegisterLogObserver(LogObserverHolder *holder);
+    void UnregisterLogObserver(LogObserverHolder *holder);
+
+}