Fixed child process signal behaviours. (#128)

* Restored signal handlers upon fork().
* Improved error handling of hpfs process kill scenario.
* Set pgid for forked processes for graceful sending of SIGINT.

src/cons/cons.cpp

@@ -519,7 +519,7 @@ namespace cons
         if (pid == 0)
         {
             // appbill process.
-            util::unmask_signal();
+            util::fork_detach();
 
             // before execution chdir into a valid the latest state data directory that contains an appbill.table
             chdir(conf::ctx.state_rw_dir.c_str());