Fix logging of pages skipped due to pins during vacuum.

The new logging introduced in 35192f06 made the incorrect assumption
that scan_all vacuums would always wait for buffer pins; but they only
do so if the page actually needs to be frozen.

Fix that inaccuracy by removing the difference in log output based on
scan_all and just always remove the same message.  I chose to keep the
split log message from the original commit for now, it seems likely
that it'll be of use in the future.

Also merge the line about buffer pins in autovacuum's log output into
the existing "pages: ..." line. It seems odd to have a separate line
about pins, without the "topic: " prefix others have.

Also rename the new 'pinned_pages' variable to 'pinskipped_pages'
because it actually tracks the number of pages that could *not* be
pinned.

Discussion: 20150104005324.GC9626@awork2.anarazel.de