Add guc to enable send SIGSTOP to peers when backend exits abnormally

I want to share a patch with you, in which I add a guc parameter 'enable_send_stop' to enable set the value of SendStop in postmaster.c more conveniently.  SendStop enable postmaster to send SIGSTOP rather than SIGQUIT to its children when some backend dumps core, and this variable is originally set with -T parameter when start postgres, which is inconvenient to control in some scenarios.

Thanks & Best Regards