src/unixsignals.cpp: fix signal handler

Fix two issues in the signal handler function:
* Use `volatile std::sig_atomic_t` for the variable that will be written
to the socket. See
https://en.cppreference.com/w/cpp/utility/program/sig_atomic_t and
https://en.cppreference.com/w/cpp/utility/program/signal

* Don't try to print anything to the log or the console in the signal
handler. We were calling `qDebug()` which seems convenient for debugging
the signal handler, however, `qDebug()` calls `gettimeofday()`, which is
not signal safe. The list of signal safe functions is here:
https://pubs.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_04.html#tag_02_04_01
which notably does _not_ include `gettimeofday()`. Unfortunately there
is virtually no way to log something safely from inside a signal handler
itself. We can log stuff once we've passed it along via Qt Signals and
Slots, but not inside the OS signal handler itself. Also see
https://stackoverflow.com/questions/53155166/is-gettimeofday-async-signal-safe-and-can-it-cause-deadlock-if-used-in-signal
and https://man7.org/linux/man-pages/man7/signal-safety.7.html

src/unixsignals.cpp

@@ -1,6 +1,6 @@
-#include <signal.h>
 #include <QApplication>
 #include <QDebug>
+#include <csignal>
 #include "unixsignals.h"
 
 SocketPair UnixSignals::sockPair;
@@ -109,9 +109,8 @@ void UnixSignals::stop()
 
 void UnixSignals::signalHandler(int number)
 {
-    char tmp = number;
-    qDebug() << "UnixSignals::signalHandler -- pass signal" << number;
-    UnixSignals::sockPair.input()->write(&tmp, sizeof(tmp));
+    volatile std::sig_atomic_t tmp = number;
+    UnixSignals::sockPair.input()->write((char* )&tmp, sizeof(tmp));
 }
 
 void UnixSignals::handleSig(QByteArray data)