Missing pg_control crashes postmaster

Hey Hackers,

If a postmaster is running and the pg_control file is removed postgres
will PANIC.

Steps to recreate:

1.) start a new cluster
2.) rm $DATADIR/pg_control
3.) psql => CHECKPOINT;

PANIC: could not open control file "global/pg_control": No such file
or directory

After the PANIC there is no pg_control. Recovery would be difficult
without a replica or a backup. Instead of crashing we can just write a
new pg_control file since all the data is in memory at the time.

There does not really seem to be a need for this behavior as all the
information postgres needs is in memory at this point. I propose with
a patch to just recreate pg_control on updates if it does not exist.

--
Brian Faherty