epoll_helper: move code to block OS signals into own helper function

before epoll can handle signals, the calling thread needs to block
them. the blocking needs to happen before any other threads get
spawned so they inherit the mask.

lib/include/srslte/common/epoll_helper.h

@@ -118,14 +118,18 @@ inline int create_tti_timer()
   return timer_fd;
 }
 
-///< Create signalfd for handling signals
+///< Blocks all signals from the calling thread
-inline int add_signalfd()
+inline void block_signals()
 {
   // block all signals. we take signals synchronously via signalfd
   sigset_t all;
   sigfillset(&all);
-  sigprocmask(SIG_SETMASK, &all, NULL);
+  pthread_sigmask(SIG_BLOCK, &all, NULL);
+}
 
+///< Create signalfd for handling signals
+inline int add_signalfd()
+{
   // add signals we accept synchronously via signalfd
   std::vector<int> sigs = {SIGIO, SIGHUP, SIGTERM, SIGINT, SIGQUIT, SIGALRM};