Don't warn about an in-progress online backup, when we're recovering from

Don't warn about an in-progress online backup, when we're recovering from
an online backup instead of performing one. pg_ctl can detect that by
checking if recovery.conf exists.

Backup label file is renamed away early in recovery, so the window where
backup label exists during recovery is normally very small, but you can run
into it e.g if restore_command is set incorrectly and the startup process
never finds even the first WAL segment containing the checkpoint record to
start recovery from.

Fujii Masao with comments by me.