Add restart_after_crash GUC.

Normally, we automatically restart after a backend crash, but in some
cases when PostgreSQL is invoked by clusterware it may be desirable to
suppress this behavior, so we provide an option which does this.
Since no existing GUC group quite fits, create a new group called
"error handling options" for this and the previously undocumented GUC
exit_on_error, which is now documented.

Review by Fujii Masao.