Use signal handler type in my_system()

For better readability.
Rename saved_dfl_sig_handler to sigtstp_handler

In my_system() function of utils/utils.c file.
Also for better readability.
Fix "Press return..." message appearing on OS X

Yes, again.

This time fix blocks SIGCHLD signal in my_system() function to ensure
that it won't be handled by appropriate handler in signals.c.  The
handler can not be defaulted, since it will cause loose of SIGCHLD
signals, hence we block it and process afterwards.

Thanks to Daniel R. (aka r1chelt).
Log errors on running external commands

This includes:
 * logging of exit code of processes that finished with non-zero
   exit code;
 * logging of waitpid() call error.
Merge branch 'fix-mac-press-return-msg'