pgsql: postmaster: Move code to switch into FatalError state into funct

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 <bertranddrouvot(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/kgng5nrvnlv335evmsuvpnh354rw7qyazl73kdysev2cr2v5zu@m3cfzxicm5kp

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8edd8c77c88e75822334ccb8376d2c151d6e5615

Modified Files
--------------
src/backend/postmaster/postmaster.c | 74 +++++++++++++++++++++++++------------
1 file changed, 51 insertions(+), 23 deletions(-)