From FreeBSD.

Index: src/cpp/session/SessionMain.cpp
--- src/cpp/session/SessionMain.cpp.orig
+++ src/cpp/session/SessionMain.cpp
@@ -1692,7 +1692,7 @@ int main (int argc, char * const argv[]) 
       
       // move to own process group
 #ifndef _WIN32
-      ::setpgrp();
+      ::setpgrp(0, 0);
 #endif
 
       // get main thread id (used to distinguish forks which occur