Fix signal handler from getting stuck in an endless loop

The signal handler for SIGINT/TERM/QUIT and, importantly, SIGPIPE tries
to write an informational message to stderr. When however stderr is
redirected to a closed pipe, this will cause (another) SIGPIPE, and in
turn the signal handler will get called again, and again and again.

Since we intend to exit rtl_fm anyways, we can just ignore this signal.

src/rtl_adsb.c

@@ -123,6 +123,7 @@ sighandler(int signum)
 #else
 static void sighandler(int signum)
 {
+	signal(SIGPIPE, SIG_IGN);
 	fprintf(stderr, "Signal caught, exiting!\n");
 	do_exit = 1;
 	rtlsdr_cancel_async(dev);

src/rtl_fm.c

@@ -246,6 +246,7 @@ sighandler(int signum)
 #else
 static void sighandler(int signum)
 {
+	signal(SIGPIPE, SIG_IGN);
 	fprintf(stderr, "Signal caught, exiting!\n");
 	do_exit = 1;
 	rtlsdr_cancel_async(dongle.dev);

src/rtl_power.c

@@ -195,6 +195,7 @@ sighandler(int signum)
 #else
 static void sighandler(int signum)
 {
+	signal(SIGPIPE, SIG_IGN);
 	do_exit++;
 	multi_bail();
 }

src/rtl_sdr.c

@@ -74,6 +74,7 @@ sighandler(int signum)
 #else
 static void sighandler(int signum)
 {
+	signal(SIGPIPE, SIG_IGN);
 	fprintf(stderr, "Signal caught, exiting!\n");
 	do_exit = 1;
 	rtlsdr_cancel_async(dev);

src/rtl_tcp.c

@@ -144,6 +144,7 @@ sighandler(int signum)
 #else
 static void sighandler(int signum)
 {
+	signal(SIGPIPE, SIG_IGN);
 	fprintf(stderr, "Signal caught, exiting!\n");
 	rtlsdr_cancel_async(dev);
 	do_exit = 1;

src/rtl_test.c

@@ -115,6 +115,7 @@ sighandler(int signum)
 #else
 static void sighandler(int signum)
 {
+	signal(SIGPIPE, SIG_IGN);
 	fprintf(stderr, "Signal caught, exiting!\n");
 	do_exit = 1;
 	rtlsdr_cancel_async(dev);