Modify elog() logic so that it won't try to longjmp(Warn_restart) before

Warn_restart has been set by the backend main loop.  This means that
elog(ERROR) or elog(FATAL) in the postmaster or during backend startup
now have well-defined behavior: proc_exit() rather than coredump.
In the case of elog() inside the postmaster, I think that proc_exit()
is probably not enough --- don't we want our child backends to be
forced to quit too?  But I don't understand Vadim's recent changes in
this area, so I'll leave it to him to look over and tweak if needed.