Unblock signals when spawning child processes.

The signal mask is inherited by children, so if we don't remove it
the shell module spawns processes which are accidentally resistant
to most signals.

see: man sigprocmask (znc uses pthread_sigmask, but they act the same)

src/FileUtils.cpp

@@ -645,6 +645,9 @@ int CExecSock::popen2(int& iReadFD, int& iWriteFD, const CString& sCommand) {
     }
 
     if (iPid == 0) {
+        sigset_t signals;
+        sigemptyset(&signals);
+        sigprocmask(SIG_SETMASK, &signals, nullptr);
         close(wpipes[1]);
         close(rpipes[0]);
         dup2(wpipes[0], 0);