Proper shutdown on SIGHUP

If a process exits and has a controlling terminal, everything under
it gets a SIGHUP by the kernel. That's the case for instance when
running a srsUE over ssh with some specific parameters. As a result,
when the ssh client is killed, SIGHUP is sent to srsUE, which will be
killed immediatelly without proper release, which means in turn that log
buffers are not flushed.
Let's do proper shutdown when SIGHUP is received so that everyone can
get log files.

srsenb/src/main.cc

@@ -424,6 +424,7 @@ int main(int argc, char* argv[])
 {
   signal(SIGINT, sig_int_handler);
   signal(SIGTERM, sig_int_handler);
+  signal(SIGHUP, sig_int_handler);
   all_args_t                         args = {};
   srslte::metrics_hub<enb_metrics_t> metricshub;
   metrics_stdout                     metrics_screen;

srsepc/src/main.cc

@@ -384,6 +384,7 @@ int main(int argc, char* argv[])
 {
   signal(SIGINT, sig_int_handler);
   signal(SIGTERM, sig_int_handler);
+  signal(SIGHUP, sig_int_handler);
 
   // print build info
   cout << endl << get_build_string() << endl;

srsepc/src/mbms-gw/main.cc

@@ -190,6 +190,7 @@ int main(int argc, char* argv[])
   cout << endl << "---  Software Radio Systems MBMS  ---" << endl << endl;
   signal(SIGINT, sig_int_handler);
   signal(SIGTERM, sig_int_handler);
+  signal(SIGHUP, sig_int_handler);
 
   all_args_t args;
   parse_args(&args, argc, argv);

srsue/src/main.cc

@@ -553,6 +553,7 @@ int main(int argc, char* argv[])
 {
   signal(SIGINT, sig_int_handler);
   signal(SIGTERM, sig_int_handler);
+  signal(SIGHUP, sig_int_handler);
   srslte_debug_handle_crash(argc, argv);
 
   all_args_t args = {};