Fix typos
Add a GUC to report whether data page checksums are enabled.

Bernd Helmle
Rename various "freeze multixact" variables

It seems to make more sense to use "cutoff multixact" terminology
throughout the backend code; "freeze" is associated with replacing of an
Xid with FrozenTransactionId, which is not what we do for MultiXactIds.

Andres Freund
Some adjustments by Álvaro Herrera
Remove `proc` argument from LockCheckConflicts

This has been unused since commit 8563ccae2caf.

Noted by Antonin Houska
Typo fix.

Etsuro Fujita
Documentation correction.

Etsuro Fujita
Fix typo in comment.

Ian Lawrence Barwick
Fix compiler warning in WaitForBackgroundWorkerStartup().

Per complaint from Andrew Gierth.