add verbosity to pg_basebackup for sync

On some recent testing, pg_basebackup -Fp was taking an annoying amount of
time to finish once the it was done copying the data. Using -v seemed to
blame this on "waiting for background process to finish streaming", based
on that being the last message on display while the delay was happening.

But it was really waiting for the syncs of the new -D dir to finish. The
attached patch adds a -v notice that it is starting to do the sync, with
the wording taken from initdb's equivalent message.

I think -P should report everything -v does, just with a carriage return
rather than a newline, but that is a larger issue.

Cheers,

Jeff