Use of backup_label not noted in log

Hi,

I've often had to analyze what caused corruption in PG instances, where the
symptoms match not having had backup_label in place when bringing on the
node. However that's surprisingly hard - the only log messages that indicate
use of backup_label are at DEBUG1.

Given how crucial use of backup_label is and how frequently people do get it
wrong, I think we should add a LOG message - it's not like use of backup_label
is a frequent thing in the life of a postgres instance and is going to swamp
the log. And I think we should backpatch that addition.

Medium term I think we should go further, and leave evidence in pg_control
about the last use of ControlFile->backupStartPoint, instead of resetting it.

I realize that there's a discussion about removing backup_label - but I think
that's fairly orthogonal. Should we go with the pg_control approach, we should
still emit a useful message when starting in a state that's "equivalent" to
having used the backup_label.

Thoughts?

Greetings,

Andres Freund