Minor clean-ups in signal.*; new function set_sig_handler
Fix for Darwin.
Before unblocking signals in the end of main()
set SIGCHLD handler to SIG_DFL and SIGALRM to SIG_IGN.
This garantees that stdout is fflush-ed().
The problem was seen on Darwin-14.0.0.
Repeat big_result_cmd regression test 10 times\