postmaster: Move code to switch into FatalError state into function

postmaster: Move code to switch into FatalError state into function

There are two places switching to FatalError mode, behaving somewhat
differently. An upcoming commit will introduce a third. That doesn't seem seem
like a good idea.

This commit just moves the FatalError related code from HandleChildCrash()
into its own function, a subsequent commit will evolve the state machine
change to be suitable for other callers.

Reviewed-by: Bertrand Drouvot 
Discussion: https://postgr.es/m/kgng5nrvnlv335evmsuvpnh354rw7qyazl73kdysev2cr2v5zu@m3cfzxicm5kp