pgsql: Improve the checkpoint signaling mechanism so that the bgwriter

Log Message:
-----------
Improve the checkpoint signaling mechanism so that the bgwriter can tell
the difference between checkpoints forced due to WAL segment consumption
and checkpoints forced for other reasons (such as CREATE DATABASE).  Avoid
generating 'checkpoints are occurring too frequently' messages when the
checkpoint wasn't caused by WAL segment consumption.  Per gripe from
Chris K-L.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        clog.c (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/clog.c.diff?r1=1.30&r2=1.31)
        xlog.c (r1.204 -> r1.205)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.204&r2=1.205)
    pgsql/src/backend/commands:
        dbcommands.c (r1.163 -> r1.164)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.163&r2=1.164)
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.16&r2=1.17)
    pgsql/src/backend/tcop:
        utility.c (r1.239 -> r1.240)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.239&r2=1.240)
    pgsql/src/include/postmaster:
        bgwriter.h (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/bgwriter.h.diff?r1=1.5&r2=1.6)