pgsql: Make the world safe for full_page_writes.

Log Message:
-----------
Make the world safe for full_page_writes.  Allow XLOG records that try to
update no-longer-existing pages to fall through as no-ops, but make a note
of each page number referenced by such records.  If we don't see a later
XLOG entry dropping the table or truncating away the page, complain at
the end of XLOG replay.  Since this fixes the known failure mode for
full_page_writes = off, revert my previous band-aid patch that disabled
that GUC variable.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.234 -> r1.235)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.234&r2=1.235)
        xlogutils.c (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlogutils.c.diff?r1=1.43&r2=1.44)
    pgsql/src/backend/storage/smgr:
        smgr.c (r1.98 -> r1.99)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/smgr.c.diff?r1=1.98&r2=1.99)
    pgsql/src/include/access:
        xlogutils.h (r1.20 -> r1.21)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlogutils.h.diff?r1=1.20&r2=1.21)