loader: keep the output and log of a game LoadExec relaunches (#1480)

* loader: keep the output of a process LoadExec relaunches

LoadExec on Windows starts a new emulator process and exits the current
one. The new process was created without a console and without the
standard handles, so everything it printed was lost: a redirected log of
a game that relaunches itself ended at the relaunch.

Hand the new process the standard input, output and error handles,
inheriting only those, and let it share the console when there is one.

* loader: flush output before LoadExec starts the new program

On Windows, LoadExec flushed the C streams only after the new process
had been created, so output the old process still held could appear
after the new one's first lines. On Linux they were not flushed at all
before exec, which discards them.

Flush the log and every stream before starting the new program.

* loader: continue the log file across LoadExec

With --printf-direction File, the program LoadExec starts is given the
same --printf-output-file and opened it anew, emptying it. Everything
the first program had logged, its LoadExec line included, was lost.

LoadExec now sets KYTY_RELAUNCHED for the program it starts, which then
appends to the log file instead of replacing it. On Linux the variable
goes into the environment passed to exec rather than this process's
own, which other threads may be reading.

* loader: show LoadExec switches on the console

LoadExec logged the program it switched to only when guest output was
logged, and then only into the log: on a console the new program's
startup output followed the old one's without anything marking where
one program ended and the next began.

Write the switch to the console as well as the log, with the program's
arguments.

src/common/logging/log.cpp

@@ -5,6 +5,7 @@
 #include "common/emulatorConfig.h"
 
 #include <cstdio>
+#include <cstdlib>
 #include <filesystem>
 #include <fmt/format.h>
 #include <memory>
@@ -45,8 +46,8 @@ std::shared_ptr<spdlog::logger> MakeFileLogger(std::string                  name
 		std::filesystem::create_directories(parent);
 	}
 
-	auto sink =
-	    std::make_shared<spdlog::sinks::basic_file_sink_mt>(path.native(), true);
+	const bool truncate = std::getenv(Log::RelaunchedVariable) == nullptr;
+	auto       sink = std::make_shared<spdlog::sinks::basic_file_sink_mt>(path.native(), truncate);
 	return MakeLogger(std::move(name), std::move(sink));
 }